DR. CLIFFORD DAHM: Examples of Managed Flow Regimes – Possible Models for the Delta?

Sacramento River; Photo by DWR

Dr. Clifford Dahm began by noting that his presentation will present possible models for the Delta, drawing on some of the work he’s done over the last 15 years relating to the issue of flows and setting flow criteria in systems that have a high degree of human impact and human management.

Dr. Dahm gave five short vignettes to spotlight the work being done to set flow criteria for rivers from South Florida, New Mexico, Texas, the Murray-Darling Basin in Australia, and South Africa.

Example 1: South Florida

Florida passed a law back in the early 1970s that required a minimum flow to be established for all major rivers, springs, and estuaries in Florida. The statute says: ‘The minimum flow for a given watercourse shall be the limit at which further withdrawals would be significantly harmful for water resources or ecology of the area.’

The statute, particularly those two words ‘significantly harmful,’ have been the crux of a lot of discussion as to what does that actually mean and how do we approach making these decisions,” said Dr. Dahm.

The Southwest Florida Management District, located around Tampa Bay, has been the most effective at setting flow criteria. “They have set flow criteria for at least a dozen of the rivers, three of their major springs, and one estuary,” he said. “I will admit readily estuaries are much more difficult.”

For guidance, they looked to the principles embodied in the 2002 article by Stuart Bunn and Angela Arthington, Basic Principles and Ecological Consequences of Altered Flow Regimes for Aquatic Biodiversity:

They have been working on this in Southwest Florida since the 1990s, and they’ve developed a process that they can apply to the rivers, springs, and estuaries. Dr. Dahm explained that it’s an iterative process that involves certain key features: “They analyze their flow regime using a building block approach, and they quickly came to the conclusion they were not talking about a flow, but were talking about multiple flows and levels,” he said. “The levels are for things like fish passage and fish ability to move either laterally or longitudinally. They have embraced and have used substantively the physical habitat simulation methodology that was developed by Jim Gore … The other aspect they have built into this is this connectivity with the floodplain, and also the groundwater surface connectivity. Much of this area of Florida is Karstic, and so there is a very strong groundwater-surface water interface that needs to be considered.”

He presented an example of their building block approach, noting that this is a typical central Florida hydrograph that’s been divided into three periods of time, from which they then go through the process of setting the flow criteria. Block 1 is the period of time when the lowest flows are expected to occur within those systems; in this part of Florida, May and June is when the predictably lowest flows are seen. Block 2 is the dry season, which is typically from January through April or May. Block 3 is the wet period, when convective thunderstorms that are prevalent as well as precipitation from dissipating tropical storms that occur in this region.

They then consider what kinds of flow requirements occur in each of these individual blocks; this is then linked to the attributes of life histories of the species of highest concern within that catchment. “Often a suite of species are selected that would include some non-native species that are in the system, key native species, often game fish, certain types of invertebrates, dominant riparian plant communities in these areas, and also some of the riparian-dependent bird species,” he said. “Then using those kinds of attributes for the life histories of those different types of species, flow criteria are developed. These have been generally a percent of flow reduction methodology that’s been applied. This is then put out for public comment, public comment is received, and then they go to their board, and the board will pass judgment on this. Then they periodically come back and review this.”

From listening to the process, you can immediately tell climate change is not in any way shape or form built into the process,” said Dr. Dahm. “However, climate variability as related to the Atlantic multidecadal oscillation is built into the process, so they are looking at wet periods and dry periods that are driven by climate, but they are not looking at directional changes to climate that might be associated with climate change.”

Example 2: Middle Rio Grande of New Mexico

The middle Rio Grande is a bit provincial,” Dr. Dahm said. “It is nowhere near the middle of the Rio Grande but most New Mexicans would like Texas to go away anyway, so we call it the middle Rio Grande. It’s really a 320 km reach in the central part of New Mexico.”

This stretch of the river is highly regulated; the Cochiti reservoir is at the upper end of this stretch, with a number of other storage reservoir above that; the Elephant Butte reservoir is at downstream end. “Basically from Elephant Butte until you get down to about Big Bend in Texas, it really no longer functions as a river; it’s simply a conveyance system for moving water for irrigation and to meet compact deliveries between the states and Mexico,” he said.

In the northern part of the middle Rio Grande, there is still some floodplain river connectivity where moderate flows will generate flooding,” he said. “As you move into the urban areas of Albuquerque, you get into a very channelized system. As you move south, that channelization continues and there is very active dewatering and it’s not uncommon for the lower half of this segment of river to be dry for multiple months during the irrigation season.”

There are two endangered species: the Rio Grande silvery minnow, which may well become a conservation reliant species; and the Southwestern willow flycatcher, a riparian bird which drives a lot of management decisions.

He presented a slide with graphs from a recent report. The upper diagram is about 20 years of data collected at 30 locations on a logarithmic scale for the numbers of Rio Grande silvery minnow that have been collected. “All I want you to see is that there was a two year period, 2002-2003, associated with a substantive drought where populations declined by about 2 orders of magnitude,” he said. “This was of great concern; it was a period of time where there was a lot of discussion about what is going to be the fate of this species. Then the drought broke and there was a period of time when there was enough water in the system to do a pulsed spring release. This fish species is very dependent on this spring flow followed by a gradual and predictable recession. This is a fish species that broadcast spawns, when that cue is in the system. And so in this period of time after this very marked downturn in 2002-2003, there was enough water that they had managed releases, and they did have this kind of spring snowmelt. Populations rebounded pretty significantly. It rebounded almost 1000x what they were pre bottleneck.

The years 2011-2012 have been the driest 24 month period in recorded history. The lower diagram is showing that in October of 2012, they did not collect any fish at any of the 30 sampling locations.

This is causing all kinds of interesting discussions with all kinds of different parties and players,” said Dr. Dahm. “The one thing that is being discussed is what is the role of flows and the lack of the spring pulses and certainly the substantive dewatering that is occurring over at least half of the reach. The other thing that also happened, concomitant with this is that we had the largest forest fire in the history of the state of New Mexico. It also burned with high severity in much of the high elevation forest, and we’ve been monitoring the water coming off these highly disturbed lands. And during this same period of time, we have now captured multiple flows that propagate through the Rio Grande where the oxygen is completely stripped out of the water column for periods of up to 2 to 3 hours at a time, so we have water quality and water quantity issues. I don’t think we are yet thinking through the possible complexity of the interactions of both a quality and quantity change within this system.”

Example 3: Texas

Dr. Dahm’s third example drew on some discussion he has had with those setting flow criteria for the seven major estuaries in Texas. “This was the byproduct of a bill passed in 2007, Senate Bill 3, and there’s some interesting verbiage within that bill,” he said. “It basically says that they must come up with a recommended environmental flow regime and that the mechanism by which they are to do this is through a collaborative process designed to achieve consensus. In addition, there is another section of the bill that says that the recommendations must be based solely on the best science available. Interesting verbiage.” Dr. Dahm noted that in speaking with one of his colleagues in Texas, they have had ‘tremendous difficulty’ coming to consensus.

They have set up a process that they are trying to work their way through,” he said. “They have a statewide environmental flow science advisory committee; this colleague of mine is on that committee, so he has that statewide perspective. They have set up bay-basin expert science teams. They are supposed to address the seasonal variability and the interannual fluctuations, so they are supposed to take both that shorter and longer term perspective. … The law calls them to do the seven major and then five minor and they are already well behind schedule.”

Example 4: Murray-Darling Basin, Australia

Dr. Dahm did a sabbatical in 2007 with Stuart Bunn and the Australian Rivers Institute. At that time, he had an opportunity to go down and visit a number of the Australian scientists that are working in the Murray Darling Basin. The Murray Darling Basin is about 2 ½ times the size of California; it is home to 3 million people and is the food basked for Australia. It has the three longest rivers in Australia, and 23 individual catchments. A large proportion of people in this area are involved with agriculture; there is also some important native biodiversity.

A lot of the issues in the Murray Darling Basin came due to a 15-year extended drought that lasted from 1994-2009,” he said. “During that period of time, there were some pretty substantive changes. For example, a lot of the bird communities associated with some of their iconic wetlands were impacted, basically the city of Adelaide’s water supply that they drew out of the Murray River became to saline to drink, and then the iconic river red gum was dying in large numbers throughout the basin.”

The crisis spurred legislative action, and the Australian National Water Act of 2007 was passed. “An interesting aspect of Australian water laws is when they pass new water law, all the old stuff goes away and then they work from the new stuff,” he said. “They established an independent authority called the Murray Darling Basin Authority and this basically group this has been charged with coming up with a flow criteria and a water management regime for the Murray Darling basin. There’s some interesting requirements in the act. For example, the 23 catchments, all of them have now been modeled with coupled groundwater-surface water hydrologic models, and it’s required that they really address this issue of the connectivity between surface water and groundwater as they make some of the planning decisions that they are going forward with.”

They have been some communication issues between the planners and some of the local people living within the basin. They are behind schedule, but they have issued the draft plan. The initial draft plan basically called for a reallocation of approximately 2.5 million acre-feet of water from farming to the environment.

That was basically taking an allocation that was in the 70% range and bringing it down to about the 50% range,” he said. “That’s what generated a lot of very strong negative feedback from some of the local communities. And so this process is ongoing; they haven’t met their deadlines but they are certainly moving forward with this. There is now a discussion and debate about whether this 2.5 MAF needs to be revised and the pressure is to revise it downward, but the process is continuing to move forward.”

Example 5: South Africa

One of the things that is interesting about South Africa is that when they moved post-apartheid, the new constitution has some interesting language in it that does relate to setting flow criteria within the rivers of South Africa,” he said. “This is a part of the South African constitution. One of the things that’s required in the constitution is that every citizen in South Africa be provided sufficient food and water, so they’ve been wrestling with what constitutes sufficient food and water. … The constitution also goes on to say that the secondary requirement is that the basic environmental needs be met, and then third, it is additional human uses of water: agriculture, industry, things of that type.”

Scientists working in South Africa have been wrestling with what this means and how to implement this and so they have developed guiding principles that they are using to set flow criteria for their river systems in South Africa.

Dr. Dahm then went through the eight principles. “The first one is that a modified flow regime should mimic the natural one, so that the natural timing of different kinds of flows is preserved,” he said. “Secondly, a river’s natural perenniality or non-perenniality should be retained. I’m getting started on an EU project funded by the French government to look at the degree of intermittency, natural and human-induced, worldwide. One of the interesting attributes of the early phases of the analysis is that when our colleague in France who’s leading this effort, actually analyzed the river channels of France, he found that 60% of the river channels in France are intermittent. 40% are perennial. So intermittent rivers, I would argue, have gotten a lot less attention then they deserve. One of the key tenets here in South Africa is if they’re perennial keep them perennial, and if they are not perennial, don’t make them perennial.

Another thing is that most water should be harvested from a river during wet months, little should be taken during the dry months, maintaining the flow conditions during the low flow periods, and then the seasonal pattern of higher base flows in wet seasons should be retained,” he said.

Another is that floods should be present during the natural wet season,” he continued. “The duration of floods could be shortened, but within limits. So one way to extract water they suggest is to actually shorten the duration of some of their higher flow events as one mechanism of getting water for human use. The next thing that they suggest is that it’s better to retain certain floods at full magnitude during the wet season and eliminate others entirely than to preserve all or most floods at diminished levels. Don’t mow the hydrograph down; leave some of that natural variability that shape channels and if you need to, take away some of the floods but leave some of that geomorphic functionality in place. And the last principle was the first flood, or one of the first floods of the wet season should be fully retained.

Another thing I would point out about first floods after a period of dryness is that they are mobilizing a tremendous amount of organic matter and nutrients, and they tend to be very stimulatory to a lot of biological processes because of the kind of material that those first floods generate after a period of dryness,” he said. “That’s a period of time when a lot of these intermittent systems then begin to flow, and there’s an awful lot of organic matter in those intermittent systems that tends to become mobilized and become part of the aquatic system.”


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