BROWN BAG SEMINAR: Flow Dynamics and Transport of Water-Quality Constituents in the Sacramento-San Joaquin River Delta

Dr. David Schoellhamer, Research Hydrologist for the USGS, earned his bachelors degree in civil engineering from UC Davis and a doctorate in Coastal and Oceanographic Engineering from University of Florida.  He has studied sediment transport in the San Francisco Estuary since 1993.  He was the lead author for the chapter on flow dynamics and transport of water quality constituents in the Delta for the State of Bay Delta Science 2016.  At a brown bag seminar held earlier this year, Dr. Schoellhamer gave this presentation flow dynamics and transport processes in the Delta.

Dr. Schoellhamer began by defining the terms flow dynamics and transport.  “Flow dynamics is basically how water moves through the channels of the Delta – how water gets around,” he said.  “Transport is how all the stuff in the water moves with the water since it can move slightly differently, so transport is how these things move.  The fancy term for the things that are moving around is constituents which includes things like salinity, heat, oxygen, nutrients, contaminants, organic and inorganic particles – all the things that are moving with the water.

There are a number of reasons why transport was important; Dr. Schoellhamer simplified it done to one constituent:  salinity.  One of the major concerns in the Delta is how far is salinity going to intrude into the Delta and where is that line of salinity.  The salinity comes from the ocean; it’s basically ocean water that’s pushing up into the Delta, and how far it goes is determined by the transport processes and how much freshwater is coming out of our reservoirs and river system to repel that salinity. That in turn will affect freshwater withdrawals from the Delta for farming and for export, as well as where certain species will be able to live.

In the paper, Recent advances in understanding flow dynamics and transport of water-quality constituents in the Sacramento–San Joaquin River Delta, they developed a conceptual framework for how transport works in the Delta.

A FRAMEWORK FOR TRANSPORT IN THE DELTA

Dr. Schoellhamer presented a simple framework for transport, explaining that the white arrows in the graphic represent transport; the two circular arrows represent mixing, as one of the things that transport does is it mixes different types of water such as ocean water and river water.  The arrow going from white to left represents the through transport of water as it moves from the watershed to the ocean through the Delta.

Transport processes are affected by a number of drivers.  One is oceanic forcing, which is the forcing that comes from the ocean and is shown as an two-way exchange between the Delta and the ocean.  The brown arrow represents the fluvial inputs, or the inputs from the watershed coming into the Delta, which are modified by the reservoirs, shown in gray.

The dam has an anthropogenic effect; it’s an anthropogenic driver,” he said.  “Humans also affect mixing in the Delta with things such as gates and barriers; we also have water withdrawals that are an anthropogenic effect on the Delta.”

Other forcings include atmospheric forcings, storms, solar radiation, wind, and also biogeochemical forcing which is basically biomass within the Delta, such as marsh grasses affecting transport, and geochemical processes that can act as a source or a sink for certain constituents.

OCEANIC FORCING

Dr. Schoellhamer started first with oceanic forcing and how the ocean affects the transport in the Delta.  “The main story here is if you get nothing else out of the talk today, it’s all about the tides,” he said.  “Tides drive so much of the transport within the Delta.”  He showed an animation to demonstrate the importance of tidal motion.

The animation shows a red dye continuously being released up at Verona and coming down the mainstem of the Sacramento River; the month is October so flows are low. The Delta Cross Channel is open so water is moving into the Central Delta fairly quickly.  Each frame has a two hour time step to it.  There is pumping taking place in the southern Delta, so there is a net flow down to the southern Delta.  Dr. Schoellhamer directed attention to the leading edge of the red dye coming down the Sacramento River.

He noted the tidal variability as the yellow plug of dye moves up into Liberty Island and down into the Sacramento River with each tidal cycle.  With each tidal cycle, there is quite a bit of tidal motion; the tidal excursion is several kilometers.

The change in colors represent a change in concentration, he said.  “Sometimes the water is the light blue, sometimes it’s sort of an orange-reddish color, and that represents changes in concentrations that take place over just a couple of hours,” he said.  “This is one of the challenges of collecting data in the Delta is that the tides change the concentrations tremendously over the course of a few hours, so you can get a water sample and a couple of hours later, you may have a very different concentration at that location.”

JUNCTIONS: MIXING IT UP

There is also quite a bit of mixing that takes place at junctions, such as Three Mile Slough and the Mokelumne River coming into the San Joaquin River.  Eventually, the dye from the Sacramento River moves into the Clifton Court Forebay and the pumps coming through Old and Middle River.  He noted that there’s a lot of tidal variability taking place down in the south Delta.  “So if you get nothing else out of the talk today, just understand the importance of the tidal motion,” he said.

Transport is driven more by tidal flow than tidally averaged flow, he said.   “This is the tidally averaged model of how transport works in the Delta, but in reality, as you can see from the animation, it is really the tides that are driving so much of the transport.  In the last ten years, we’ve gained a much better appreciation for how transport works in the Delta due to this tidal variability.”

Dr. Schoellhamer presented a graphic showing an example of dispersions at junctions, this example showing where Georgianna Slough junctions with the Mokelumne River.  The graph shows the path of drifters released at this junction.  “They often get trapped here and what numerical modeling shows is a recirculation zone at this junction,” he said.  “So for instance, water coming out of Georgianna Slough carrying a constituent of concern, some of that will get caught up in this recirculation zone for awhile and then leave the recirculation zone.  This causes a net dispersion, a smearing of that signal from Georgianna Slough into the Mokelumne River.”

He noted that simple one-dimensional models aren’t able to simulate this dispersion that takes place at the junction because they assume the junctions are well mixed; multi-dimensional models better represent mixing at the junctions.

TIDES AND TRANSPORT IN THE DELTA

It is the advances in acoustic and optical instrumentation that has greatly increased understanding of the effects and mixing by the tides.  There are acoustic instruments that can measure velocity continuously; a large network of those instruments is maintained by the USGS.  There are also optical instruments that can continuously measure a wide range of water quality parameters in the Delta.  “This tremendous increase in the available technology has really increased the data we can collect and better improve our understanding of the tides and how tides affect the Delta,” he said.

Dr. Schoellhamer presented a graph for Mallard Island at the junction at where the Delta essentially becomes the Bay.  He noted that that the gray line represents the tidal flow averaging about 300,000 cfs; the tidal averaged flow shown in red is a couple order magnitudes smaller than that.  “So we have very small tidally averaged flows, but very large tidal flows,” he said.  “A tremendous amount of tidal energy and water movement with the tides compared with the tidally averaged situation.”

DELTA OUTFLOW

Comparing measured flow with calculated Delta outflow, Delta outflow can be inferred from the flow gauges; there are also calculations of Delta outflow from DWR’s Dayflow, which is essentially a conservation of mass of water calculation of what Delta outflow should be.  “What we find is that for high flows, in this upper part, or for most flows, the conservation of mass and the measurements do a really good job comparing with one another,” he said.  “However, for the low flows up to 10,000 cfs, shown in the lower graph, there’s a lot of scatter that takes place there, and one of the reasons for this is the differences in water levels in the Delta.”

Changes in the ocean forcing can cause slow changes in water level; as wind blows into the Delta, it can cause a setup of water level in the Delta and affect flow in and out of the Delta as the water level changes, he said.  “For these low levels of freshwater discharge, the calculations assume what us engineers call a ‘rigid lid’ – that the water level is constant when in fact, the mean water level in the Delta varying and this causes changes in the flow that we can observe through the network of sensors that we do have,” he said.  “So for management, one implication of this is when you have the low flows, which are the most critical flows to manage well, we have the most uncertainty in what that outflow actually is.”

TIDES AND TURBIDITY

Tides also affect other things, such as turbidity.  Dr. Schoellhamer presented graphic showing Cache Slough, which is where they typically see the highest turbidity or suspended sediment concentrations in the Delta.   “In the studies that we’ve done, we’ve found that the mechanism is that in the Cache Slough area, there is a network of dead-end channels and very low freshwater flow.  As the tides move up into these fairly shallow channels, their character changes.

There’s an asymmetry in the tides; the floods and the ebbs are not equal,” he continued.  “We actually see greater flood velocities than ebb velocities.  These greater flood velocities favor sediment transport in the landward direction.  The arrows show a net transport of sediment up into the Cache Slough area where this sediment becomes trapped.  One reason it becomes trapped is that we have limited tidal excursions up there.  The tides only move back and forth so many kilometers, so a sediment particle that starts an ebb tide in the upper Cache Slough doesn’t get all the way out to the Sacramento River.  By the time the tide stops, it’s only moved as far down as lower Cache Slough, the tide reverses, and moves that particle back upstream.  So we have this trapping mechanism in Cache Slough that traps this mass of sediment there.”

With repeated cycles of erosion and deposition, we have higher concentrations of suspended sediment concentration and therefore turbidity up in Cache Slough, which is considered a favorable environment for creatures that like turbidity, such as the Delta smelt,” he said.

Looking at the historical ecology work that SFEI has done, the isolated dead end sloughs were once a prominent feature of the Delta; they create what we know now is desirable habitat.  “There were a tremendous number of these small, dead-end sloughs with large lengths and therefore relatively small tidal excursions that were ripe opportunities to trap sediment and turbidity,” he said.  “Except for this area of Cache Slough that we have now, the Delta today is really just a series of interconnected waterways with very little isolation that is present.  So the one thing that we observe from this particular work is that creating these isolated channels are a feature of the Delta that does not exist may be useful for restoration.”

THE EFFECT OF SEA LEVEL RISE ON TRANSPORT

Sea level rise also affects transport in the Delta; as sea level rise, certain processes will become more prominent, he said.  “One of those is stronger gravitational circulation, essentially the landward flow of denser water along the bottom; that increases with theoretically the water depth cubed, so as you increase sea level and increase water depth, you get stronger gravitational circulation.  The result of this is that the upstream movement of salt will become greater as sea level rises, and therefore more freshwater is needed to maintain the salinity fields at the positions it is now, (basically the X2 standard).  There would also be less water for diversions.”

Dr. Schoellhamer noted that the mechanism of using reservoir releases to control salinity in the Delta is anthropogenic feedback to take the oceanic conditions and sort of move those up to the watershed.  “Normally that is only done through atmospheric forcing, but as humans, because we regulate the reservoirs to counter the oceanic forcing, we’re regulating the watershed and fluvial environments based upon the ocean, so we create this sort of additional feedback loop from the ocean to the fluvial and watershed systems.  This is an example where we have several interacting drivers: the oceanic forcing, the anthropogenic forcing and the fluvial forcing, all interacting with one another.”

DELTA OUTFLOW AND THE SAN FRANCISCO BAY

He then gave an example of how the Delta affects San Francisco Bay.  “To give you the extreme example of this, I’ll go all the way down to South San Francisco Bay, where we measure the movement of sediment at the Dunbarton Bridge near San Jose.”

He presented a plot of monthly Delta outflow during the spring and millions of cubic acre-feet versus the monthly values of sediment flux in kilotons at the Dunbarton Bridge for several different water years.  “What we see for these two sets of water years is a fairly good relation of increasing sediment flux out of South San Francisco Bay as Delta outflow increases, so there’s a linkage between Delta outflow and sediment movement in South San Francisco Bay,” he said.  “There are two sets of data here because with the drought, water years 2013-2016, the sediment concentration in the south Bay has actually increased, possibly due to less of this flushing taking place, so that’s why we have the two different sets of lines here for these two different periods: earlier drought and later drought.”

It has been hypothesized for decades that there is a clear relation between flushing of South San Francisco Bay increasing with increased Delta outflow in the spring, said Dr. Schoellhamer, presenting a simple conceptual model to illustrate how it is thought to work.

This white box is what we consider a control volume for lower South San Francisco Bay, and there are a number of sediment fluxes going into and out of this control volume.  There is erosion and deposition, there is gravitational circulation that takes place between the Central Bay and South Bay that transports sediment in and out of South Bay; generally the circulation is in near the bottom where the sediments concentrations are greater so this causes a net transport of sediment into lower South Bay.

For the lower South Bay, the sediment concentrations tend to be greater than Central Bay so there is a dispersive flux out of lower South Bay towards the Central Bay,” Dr. Schoellhamer continued. “If we have an equilibrium situation where the sediment concentrations are relatively constant and these fluxes are all in equilibrium, and then we perturb these fluxes by having a Delta outflow event and decreasing the salinity in Central Bay, that decreased salinity decreases the density difference between Central and South Bay, and the result is we have less gravitational circulation taking place.”

So we perturb this system by decreasing the gravitational circulation when we have a Delta outflow.  The net effect of this is with the gravitational circulation directions, we have less sediment coming into south bay.  With less sediment coming in and disturbing the equilibrium, the concentration in south bay goes down and the result of that is we have less deposition and less dispersive flux out, and then south bay compensates to establish a new equilibrium.  Now if you’re a restoration project in the south bay, such as the South Bay Salt Pond project, a decrease in deposition may be of concern or interest to you.”

This is an example of how the Delta affects the Bay ranging all the way down to South Bay, so there are certainly downstream effects of what happens in the Delta,” Dr. Schoellhamer said.  “What happens in the Delta doesn’t stay in the Delta, is the bottom line.”

FLUVIAL DRIVERS

The next main driver of transport are fluvial processes.  He presented a picture of the boat ramp at Hogback Island in the Delta, explaining that during high tide, there is fluvial forcing that increases the water level.

We launched the boat just fine at the boat ramp and get back and we all had looked at the tide tables, but realized that there’s a high tide when we get back in the afternoon.  This is the boat ramp at the high tide with a few inches of water on top, so we do have these interacting drivers of high water due to the fluvial inputs in addition to the sea level and the tidal signal that is present.”

IMPORTANCE OF CONTINUOUS DATA MONITORING

There have been advances in technology for measuring flow and some constituents which have been important for understanding the tidal environment with tides sloshing back and forth, as well as riverine or fluvial signals coming into the Delta.  Dr. Shoellhamer presented an example of nitrate samples collected on the San Joaquin River.

A number of years ago, the samples were collected every 3 or 4 hours – a lot of samples collected over a short amount of time.  Dr. Shoellhamer pointed out how the figure on the left shows a noisy linear increase in nitrate concentration, but when matched with a continuous sensor collecting 15-minute data as shown on the right, what looked like a linear increase in concentration actually has quite a bit of pattern to it with the natural variability in the river.  “This demonstrates that even in the riverine environment, the importance of collecting the continuous data to observe these rapid changes that take place on the order of an hour or two, and the power of the continuous data compared to the discrete water sample type of data.”

FIRST FLUSH

Dr. Schoellhamer said that with the improvements in monitoring over the last ten years, one of the things they’ve focused on is the effects of the first flush.  The first flush occurs with the first significant rainfall that occurs in watershed; after 8 inches of rain or so, all the rain that falls will runoff the watershed, bringing the first flush of sediment, material, contaminants, and everything else with it.  A lot of sampling over the past ten years has been focused on the first flush.

He presented a plot showing turbidity; the black line is at Freeport up in the riverine part of the Sacramento River, the red line is at Rio Vista down in the lower Sacramento River, and the blue line is turbidity at Mallard Island as that pulse of sediment moves from the river down through the Delta.

Several observations: the duration of the first flush is fairly short, typically about a week or two.  The bottom plot is the tidally averaged discharge,” he said.  “The first flush lasts about a week or two, so if you’re sampling on a monthly basis, for instance, you may completely miss the first flush or just get one point during it, so it requires a lot of sampling during a short amount of time.”

There is also a rapid change of the river input, he said.  “The river input isn’t constant, but even at Freeport you can see the spikes which is probably inputs from different watersheds up in the upper Sacramento River being transported down the Sacramento River and showing up at Freeport or the downstream end of the river environment, so the river isn’t a monolith that’s putting out a constant concentration.  It can vary tremendously too.”

Dr. Schoellhamer noted that even though there is a first flush flow pulse occurring, there still is tidal variability.  There is also diffusion taking place; as the pulse comes down through Rio Vista and out to Mallard, the tidal sloshing back and forth and the mixing at junctions takes the peak concentrations and spreads them out.  “So what we wind up with at Mallard Island is a pulse that takes place over several weeks as opposed to just one or two weeks, so it tremendously spreads out the pulse of material coming down during this first flush,” he said.

Another question about fluvial inputs is what were historical conditions like.  Dr. Schoellhamer worked with a graduate student who looked at historical flow and sediment input data to provide better estimates of flow.  The data he used was from old records created by a physician who collected them daily from the Golden Gate Bridge over 30 years.

With the Golden Gate data, the freshwater outflow affects the tides, so by calculating certain frequency tidal constituents and how those change in time, one can back calculate what the freshwater flow is and extend the instrumented record back in time to when water level was first collected at the Golden Gate beginning in the 1850s and 1860s,” he said.

Dr. Shoellhamer noted that the plot on the left is the flow with water year days along this axis and years from 1850 to 1930.  He pointed out the magnitude of the 1862 flood, which is the biggest flood seen in modern times.  “It’s a cautionary note that there’s always a bigger flood out there that could happen,” he said.  “At least in this instrumented record, the biggest flood on record is the 1862 flood.”

For sediment inputs, they used bathymetric change data collated by Bruce Jaffe at the USGS to estimate then sediment inputs to generate that kind of bathymetric change.  “From 1850 to 2000, that sediment transport is largely determined by these very few large flow events throughout this record, and the magnitude of the 1862 flood, which was the largest flood and also had a tremendous erodible pool of sediment due to hydraulic mining,” he said.  “For the past several decades, there’s been relatively little sediment input as that erodible pool has diminished.”

For the Delta, work has been done looking at the Interagency Ecological Program’s monthly data for total suspended sediment at five sites in the Delta; using the average values, a break point analysis was done which finds the best fits of lines and breakpoints where there’s a sudden change in the data.   What was found that there were two significant breakpoints in 1983 and 1998 where there were large step decreases in suspended sediment concentrations centered around those years, which happen to be two of the bigger flow years with the El Nino flows.

What we observe here is a decrease in sediment concentration after these really, really big flows,” he said.  “The result here is basically sediment being washed out of the system, decreasing the erodible pool of sediment and therefore decreasing the available sediment concentrations.  Another thing we found is that since the 1998 El Nino, that sediment concentrations in the Delta have continued to decrease with time through the present day, and there’s been about a 50% decrease in the suspended sediment concentrations in the Delta since the 1998 El Nino.”

One question that often comes up is will this decreasing trend in sediment continue – will the Delta waters get clearer and clearer or will they stay at roughly the turbidity they are at now?  Dr. Schoellhamer presented a graph showing different estimations under different climate change scenarios.

This is the historical record of sediment decrease, and these are the two options,” he said.  “We consider one to be basically our constant sediment relationship with flow and another a continued decrease of sediment concentration shown here.  The question is often which one of these scenarios will happen.  What we know through the work we’ve been doing that the sediment supply is in fact decreasing down the rivers and that the watershed in the estuary adjusts to this decrease in supply.  We also know that these rare large floods transport a lot of sediment.  The big floods are really important for moving the sediment; we observed these recent step changes in bed elevation and suspended sediment and these are associated with the largest floods that have been experienced since hydraulic mining.”

The hypothesis we have to perhaps explain the future is that it is likely that the estuary and watershed are still adjusting; the concentrations for instance in the Delta can still continue to decrease, but the adjustment will happen as these steps that occur as we have larger and larger floods then previously experienced during the adjustment period,” he continued.  “Another way of saying this is that larger and larger floods are needed to exceed geomorphic thresholds – the thresholds to move a tremendous quantity of sediment, and that between these larger floods, we have periods of equilibrium and these periods of equilibrium, relative tranquility if you will, will get longer and longer with time.”

ANTHROPOGENIC AND ATMOSPHERIC DRIVERS

Dr. Schoellhamer then turned to the anthropogenic and atmospheric drivers of transport in the Delta.  He noted that from their previous work considering the causes of reduced sediment supply, one of which is the diminished hydraulic mining pulse or basically less erodible sediment in the watershed.  Another cause is that the reservoirs trap sediment and essentially cut some of the Sierras off from the watershed in terms of the sediment transport.

A colleague at USGS found another mechanism recently with some modeling results as part of the CASCADE project; they developed a watershed model of sediment for the watershed, and they looked at different scenarios including warmer temperatures and what they found is that warmer temperatures have been causing as one would expect more rain and less snow.  “One of the effects of the changing hydrology is that these peak flows that are so important for sediment transport have been decreasing, so these decreased peak flows have contributed to the reduced sediment supply that we’ve been observing.  So we have not only anthropogenic causes of the sediment supply decrease, we have atmospheric causes also with the changing climate.”

Another type of the anthropogenic and atmospheric forcing is the timing of flows.  He presented a slide showing outflow estimates on the vertical axis and water year days on the horizontal axis.  The red line represents the present day, 1968 – 2008, and the blue line is for the 1800s.

This is actually based on the instrumental record collected at the Golden Gate Bridge, so again, measure the tides, back calculate the freshwater flow, and what we get are these two hydrographs here,” he said.  “The observation from this is the modern inflow is about 35% less than the historical inflow – basically the area under the blue curve is greater than the area under the red curve.  The flow peaks are earlier than previously, about 100 days earlier in this century than in the 1800s.  This is based on an instrumented record and calculations from the instrumented record, not numerical modelings or assumptions.”

BIOGEOCHEMICAL FORCING

Lastly, Dr. Schoellhamer turned to biogeochemical forcing.  One example is marsh restoration, which in some ways is an anthropogenic driver on the Delta system; planting marshes can affect the transport by changing the bathymetry and the topography of the Delta which affects how the tides propagate through the Delta and therefore effect mixing and transport.

Marshes also capture suspended sediment; they are effective sediment traps, and so are fluvial inputs of sediment that may increase marsh restoration.  “One of the ideas is to capture sediment so that it may increase the efficiency of sediment trapping within the Delta,” he said.  “The marsh can also act as a geochemical force or sink.  For instance, the big concern is with methylmercury; the marsh can be a source of methylmercury for the Delta.”

Another biogeochemical is the invasive aquatic vegetation that’s been spreading throughout the Delta.  Since about 1998, there’s been a general continuing decrease in suspended sediment.  “One hypothesis that we have is that the aquatic vegetation which has been growing during that time has contributed to that decrease in suspended sediment,” he said.  “We did a study looking at the long-term trends of turbidity in the Delta and how close they are to submerged aquatic vegetation and what we found was that at sites where there’s a greater decrease in turbidity and suspended sediment generally have more aquatic vegetation.  Through some estimates in the paper, we were estimating that 21-70% of the declining turbidity trend is attributable to invasive vegetation expansion.”

Dr. Schoellhamer noted that there is a new USGS project he is working on that is evaluating how aquatic vegetation affects turbidity and traps sediment, and therefore affects the sustainability of marshes; is the sediment is moving into the marshes or moving into the vegetation as opposed to moving into the marshes.  It is an internal USGS project that is trying to evaluate the effect of the vegetation on sedimentation processes.

INFORMATION GAPS IDENTIFIED IN THE STATE OF BAY DELTA SCIENCE PAPER

Dr. Schoellhamer said that the paper addresses information gaps, and one of the things he wanted to point out is that the continuous flux based monitoring has been very helpful for understanding how things move through the Delta.  “We need to continuous model to evaluate tidal processes because they are tremendous to the transport that takes place due to the tides, as well as episodic events.  When you have big flows like this, it’s really difficult to get instruments out now, but it’s good if you have the instruments already in place.  The continuous monitoring is very helpful for getting the episodic events and the tidal variability.”

Flux based measurements are important because typically a water sample tells you a concentration of your favorite constituent – how much mass per unit of volume is there,” he said.  “The flux based measurements, which include the water discharge, tell you how much mass of that constituent is passing a cross section, and so with that, you can trace how much mass is coming into the Delta, coming out of the Delta, or coming into a restoration project and out of a restoration project and with that, you can develop budgets for constituents of whatever constituent you have of concern.  Being able to make the unit of quantification flux as opposed to concentration is very helpful for developing these constituent budgets as well as doing things like TMDL calculations.”

Monitoring should be done on the landscape scale, he said.  “The watershed, the Delta, the Bay and the ocean are all connected with one another through the natural progression of flow, through tides, through anthropogenic effects,” he said.  “Throughout my career, I’ve found that in order to understand what happens in the Delta for instance with sediment, you need to look up in the watershed and understand what’s happening in the watershed, so these environments are well connected.  This needs to take place over a long-term, certainly greater than the decadal time scale.  For instance we’re having at least a one in ten year event now, and so a lot of long-term sampling is needed to capture these types of important events like we’re having today.”

QUESTIONS AND ANSWERS

Question:  One of the conclusions you made was that warming temperatures, more rain less snow, how does that translate to lower in the peak flow?  Because it seems to me it’s the opposite.   Can you explain how that mechanism is working?

The best answer would be from a Michelle Stern paper on that topic,” said Dr. Schoellhamer.  “The mechanism from my reading of the paper is that basically with more rainfall, less snow, the input of moisture is being spread out over a greater period of time, and so the peak flows there tend to be less.”

Question: The reduction in the sediment source for the Delta.  I understand the effects of reservoirs and the depletion of the mine tailings.  How about the leveeing the system?  We now have levees up and down the system rather than sheet flow over larger floodplains.  How much of that has immobilized sediment in the valley and kept it in place?

In the papers we’ve done, that’s certainly one of the contributing factors, that as we stabilize the levees for obvious reasons, one of the things that does is it prevents meandering of the rivers, so the river that used to cut into its banks mobilizing sediment and bringing that downstream,” he said.  “The papers have the exact numbers, but it’s somewhere like about half of the Sacramento River banks have been leveed in the past or rip rapped in the past 50 years to stabilize them and that again prevents the meandering and decreases the sediment supply.  You can think of the Yolo Bypass as being a floodplain that in a geomorphic sense, at some point, the river would cut into that and remobilize some of that sediment in the old floodplain, but we obviously aren’t going to let that happen anymore.”

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