South Fork of the American River; Photo by DWR

ROBERT SHIBATANI COMMENTARY: The Final Demise of Unimpaired Flows

We can Thank Climate Change…

Guest commentary by Robert Shibatani

Some ideas have the uncanny ability of persevering! Like a dirty old coin, they just keep showing up! In water resources management, the concept of unimpaired flows has been around for years. By definition, they are generally acknowledged as those instream flows that would occur in rivers and their tributaries in the absence of dams and other diversions. These flows have been a consistently important focal point for environmental groups, particularly those opposed to dams. But fisheries biologists, river corridor managers, naturalists, recreationalists, and community river activists have also been allied to this flow concept.

The value of unimpaired flows, it has been argued, was that resource assessments founded on the “natural” hydrology of the stream network would foster an unbiased and more accurate evaluation of the impacts of past, present, and future water regulation and activities on stream networks as a whole. While it is true, they do serve as a realistic historic benchmark, so what? There are countless things in our contemporary environment that were not here even 100-years ago. Does that, in and of itself, mean that they must be preserved through specific conditions set out in active legislation and regulation?

Unimpaired flows are nothing more than a look-back into the past; some melancholy travel back in time to when things were the way they, well… once were. But that was then, and this is now. The reality is that humanity has since grown well past the era of free-flowing rivers. Human evolution has been vibrant, innovative and expansive; our station in life has not been listless. Our impact on water resources through the use of dams and reservoirs has given us hydropower, flood protection, water supplies for homes, agriculture, industry, coldwater to enhance and extend anadromous fish life-cycles, and added recreational opportunities, to name but a few.

Despite all of this, the concept of unimpaired flows has endured (much longer than reasonable in my opinion). While it was argued that unimpaired flows would allow resource assessments to be founded on the “natural” hydrology of the stream network, this had fundamental drawbacks.

As noted by Ann Willis in late 2018, there are four questionable assumptions that form the basis of many unimpaired flow strategies:

1. Today’s unimpaired flows would support anadromy and native fish.
2. Water temperature and water quality problems are changed by instream flow management.
3. Watershed-scale models provide detailed insights for managing localized water temperature and water quality.
4. “Best available data” will be enough to develop effective, watershed-wide regulatory management.

Baseline hydrology is the key. Unimpaired flows do provide fixed frames of references for flow rates. This cannot be denied. But it also assumes that the hydrologic baseline does not change. Reference points are just that, references. They can’t be dynamic; they must be stationary. Under a rapidly changing climate, the most significant alteration to system hydrology is that of a changing baseline, the very same baseline that unimpaired flow advocates were saying was so important to assess man’s influence on river flows.

But even before climate change became the issue it is today, hydrologists were arguing that man-made river systems had changed so notably, some insist irreparably, that it would be futile to try and replicate natural run-of-the-river systems. The fact of the matter is that we live in a world where the majority of the large rivers are operated as carryover-based systems, that is, dammed and reservoir controlled. Why attempt to return to a flow regime that, today, with the increasing effects of a changing climate, grows farther away from the original baseline hydrology of centuries ago?

It has been stated within the water purveyor community that, “The term ‘unimpaired flow criteria’ could soon become a very bad “word” amongst the agricultural community.” Point of fact, among many practicing hydrologists, it has always been a very bad word…and with climate change it just became much worse…

The only outstanding question is whether water regulators will also recognize it as such…

About the Author: Robert Shibatani is a consulting hydrologist and water industry advisor with over 35-years combined experience in research, environmental analysis, consulting and expert witness litigation across the globe. He is CEO of The SHIBATANI GROUP, Inc. based in Sacramento, California. <>

Note:  The views expressed in this commentary are the author’s views, and should not be attributed to Maven’s Notebook.



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