By Ann Willis and Andrew Nichols
In the early fall of 2012, an unusually large number of Chinook salmon were returning to the Klamath River, straddling the California-Oregon border. Many of those fish were expected to swim upstream to the Shasta River, prompting emergency actions to increase stream flows in the upstream tributary.
When Chinook enter the Shasta, they pause in pools before heading further upstream to spawn. The Shasta naturally runs low this time of year, and irrigation diversions to support the region’s cattle ranching further reduce flows.
With few fish, even low flows can provide enough pool habitat. But when large numbers return, low-flow pools fill up with fish more quickly. In these conditions, fish can rapidly deplete dissolved oxygen, even if plenty of oxygen is flowing into the pools.
In 2012 and 2013, the Shasta River Water Transaction Program worked with agricultural landowners, resource agencies, the local watermaster and others to coordinate voluntary flow contributions to support fall-run Chinook during the critical few weeks near the end of the irrigation season. It was a remarkable act of stewardship that defied the conventional farms-versus-fish showdown. But was the sacrifice of agricultural water worth it?
The benefits of such “environmental flows” are often questioned, but seldom quantified with scientific rigor. Measuring the outcomes is critical to furthering participation in voluntary agricultural and urban water transfers, particularly during dry times.
The UC Davis Center for Watershed Sciences collaborated with scientists from the Nature Conservancy and Watercourse Engineering Inc. to see if we could quantify the potential water quality risks and potential ecological benefits of the Shasta River water transfer. Could the salmon actually suffer for lack of oxygen and, if so, would the additional water from volunteers make a difference?
We found that low flows do not always pose an oxygen risk to salmon in pools, even when fish fill the pools to capacity. But when there is a problem, more water helps prevent harm to fish.
The study, recently published in the Journal of Water Resources Planning and Management, drew connections between the physical action – increasing stream flow – and ecological outcomes.
To make those connections, we defined relationships between stream flows, fish numbers, dissolved oxygen levels and habitat capacity in pools where Chinook congregate.
First, we tested methods to quantify the amount of pool habitat available. We used rating curves, a common tool for measuring stream flows, but a new application for evaluating aquatic habitat. The method is more cost-efficient and provides more information than traditional aquatic habitat surveys. While surveys provide a few snapshots of pool habitat over weeks, the rating curve approach allows you to quantify habitat changes almost minute by minute.
We found a strong relationship between stream flow and pool volume.
Next, we looked at how changes in pool volumes affected the amount of oxygen available to fish, and whether adding water made any difference.
We found that when flows were low, water temperatures were high, and pools were filled with fish, the fish could create their own water quality impairment simply through respiration.
But we were surprised to find that even a small increase in stream flow (about 10 cubic feet per second) could improve water quality by moving dissolved oxygen through the pools more quickly. Increasing stream flow also increased the size of pools to support more fish – without degrading water quality.
The implications of the stream flow experiment are extensive. Water markets, water trusts and other mechanisms compensate water-rights holders for providing environmental relief. Knowing the environmental value of water can ease tensions between competing users.
The study showed a way to quantify the environmental benefit of water contributions. This accounting method brings flexibility and transparency to a potentially contentious decision. It helps water-rights holders and resource managers decide whether fish would benefit from increased stream flows and make informed decisions about the value of their water.
Ann Willis, an engineer with the UC Davis Center for Watershed Sciences, developed this project as a consultant for Watercourse Engineering Inc. Andrew Nichols is a geomorphologist with the center.
Willis, et al. 2015. Instream flows: new tools to quantify water quality conditions for returning adult Chinook salmon. Journal of Water Resources Planning and Management
Shasta River Water Transaction Program. Cooperating on Streamflow for Fish and Ranching in Times of Need