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Scott River Flows
December 3, 2010
Ridin' Point

- a weekly column published in the Siskiyou Daily News


Scott River: A recent regional fisheries column has once again dragged out the old myths about the impact of agricultural irrigation on the Scott River. Here are the facts. http://users.sisqtel.net/armstrng/facts_about_the_scott_river.htm 

As a tributary, the Scott River provides only about 4% of the full natural annual flow to the Klamath River. According to the Siskiyou Co. Annual Crop Report, the Scott Valley experiences 22 inches of annual precipitation, (30 inches of snowfall.) This can vary widely with the east side averaging 12-15 inches and the southern mountains receiving as much as 60-80 inches. The Scott River has no dams or reservoirs. Historically, there was storage of cool water in 28 high mountain lakes located on the Klamath National Forest. As many of these are now located in Wilderness, they have fallen into disrepair as functional storage structures. With 292 acres of surface, one foot of additional water storage in these lakes could provide 5 cfs of instream flow for a 30 day period in the summer. Storage for summer flows in the Scott is primarily in the form of natural snowpack. Snow can hold the water into late spring when it melts to feed the streams.

Summer and fall flows in the Scott vary from year to year, but are largely controlled by the precipitation and snowpack of the prior 12 months. (Drake, Tate and Carlson) From 1951-1998, there has been a decrease in the water content of the snowpack in the area, particularly in the western mountains. There has been a correlating decline in fall river flows over time.

The number of irrigated acres in Scott valley has not changed substantially since 1950. (It was 34,100 acres in 1988 and 31,800 in 2000. The total watershed is 520,968 acres, so irrigated agriculture represents only 6% of the land.) Methods of irrigation, (flood, wheel lines, pivot wheels,) have changed over the years. In 1968, when water was more commonly diverted for flood irrigation, 86% of irrigation was through diversion of surface water, 2% groundwater and 12% mixed. In 2000, 48% was surface water, 45% groundwater and 7% mixed.

Understanding the affects of irrigation on flows is complex. Flood irrigation diverts water directly from the stream, Other methods rely on water pumped from the wells. Summer in Scott Valley can see ambient air temperatures in the 90-100 degree F. Different methods of irrigation can affect the amount of water consumed through evaporation, plant transpiration and how much is returned to the soil to feed subsurface flow and to recharge the aquifer. For instance, pivot wheels are thought by the state of CA to be the most efficient method of delivering irrigation water. They can have a high evaporation rate, while less efficient flood irrigation returns water not directly consumed in evapotranspiration to the streams as tailwater. Groundwater use, although not taking water directly from the stream, can intercept subsurface flows.

Photos are often cited as documentation that irrigators are “sucking the river dry.” In many areas of the valley, heavy gravel sedimentation has raised the bed of the tributaries above that of the mainstem Scott. In some areas, historic mining has caused build up of  gravels. In Kidder Creek, an historic fire upstream cause mass erosion resulting in gravel deposits. In such cases, water passing through seeks its own level. The river will flow through the gravels where it has accumulated and resurfaces on the other side.    

In many areas of the State “conjunctive use” is the method of water storage. This is where water is injected or percolated down into the ground in concentration in order to recharge the aquifer as a storage receptacle. According to a presentation by Dr. Thomas Harter, the average annual discharge in the Scott Valley watershed is 615,000 acre feet of water. This is more than the groundwater basin can hold (400,000 acre ft. capacity– U.S. Geological Survey.) Of this, the Department of Water Resources has estimated that agriculture uses only 70-90,000 acre ft. annually. In general, any groundwater loss is recharged within a year. It is reasonable to expect better system responses with a more sophisticated understanding of the groundwater in Scott Valley, renewed use of the historic mountain lakes, and downstream movement of some of the gravel build-up.

Scott Valley farmers and ranchers have been working on salmon “restoration” and conservation projects for decades. The Northern California Coastal Coho Salmon is listed both on the federal and the State level as a “threatened species.”  Preliminary Dept. of Fish and Game Spawning Run Estimates for coho from 2006/07 - 2009/10 illustrate that run counts in Scott Valley are, by far, among the highest in the State. Our farmers and ranchers are obviously doing something right.

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