Slightly more than 3 kilometers above Capay, there is a dam across Cache Creek that is the heart of the water delivery system for Yolo County agriculture.

Capay Diversion Dam is a simple concrete structure, 474 feet long and 15 feet high, built in 1914 and showing some wear and tear with age. In 1994, it was modernized with the addition of an inflatable dam, billed as the “longest single bladder dam in the world,” on top of the old concrete.

The inflatable dam, which can be raised or lowered in 30 minutes, raises the dam 5 feet and increases its ability to divert water. The dam belongs to the Yolo County Flood Control and Water Conservation District (http://www.ycfcwcd.org).

Capay Diversion Dam takes water from Cache Creek and sends it throughout Yolo County by a network of 175 miles of canals, which start with the Winters Canal and the West Adams Canal at the dam. The canal system is capable of transporting fish long distances.

Willow Canal, along the south boundary of Yolo County (near Davis), is part of the network and actually spills on occasion into Putah Creek. Thus, when inland silversides, introduced into Clear Lake in 1967, started their march through California, one of the first places they were found outside of Clear Lake, besides Cache Creek, was Putah Creek (in the early 1970s).

While Clear Lake is one of the oldest lakes in the world, it is also managed as a reservoir to deliver water to Yolo County. The “extra” water flows downstream in summer and is diverted at the Capay Dam. Thus, except during years of drought, Cache Creek above the dam had more flows in summer than it did historically. These summer flows were enhanced even further by the construction of Indian Valley dam and reservoir in 1976 on the North Fork of Cache Creek.

The entire volume of this reservoir is available for use by the district after flowing down to the Capay Diversion Dam. The recreational rafting that is possible in Cache Creek Canyon in summer is mostly on water that is released from Indian Valley Dam. This water is both more plentiful and colder than the original creek water.

One result of having higher flows in summer than in winter is that carp and other large fish from Clear Lake become stranded in big pools in the creek in the winter. These fish become vulnerable as prey for bald eagles that winter now in the canyon. The eagles, in turn, attract birders and naturalists by the carload.

The reverse flow regime supports a fish fauna of the creek that is a curious mixture of native and alien species. One of the more abundant aliens is smallmouth bass, a favorite game fish, which tends to eliminate native fishes from streams where it is found. Somehow, the natives have managed an uneasy truce with this predator, so hardhead, pikeminnow, sucker, speckled dace, hitch and other species are fairly common, as least for now.

Below the Capay Diversion Dam, conditions are much less favorable for native fishes. In pools immediately below the dam, native pikeminnow, suckers and dace are still common. But habitat becomes increasingly less suitable downstream. Not only do flows rapidly decrease downstream, but the channel becomes braided through miles of past and present gravel mining operations.

In the shallow channels, the dominant fish in summer is the red shiner, a recent invader from the east. At times, it so abundant that it is likely suppressing populations of the remaining native fishes in the lower creek (hitch, pikeminnow, sucker, speckled dace) through competition and predation on larvae.

The shiner was brought to Central California (under protest!) by bait dealers, who were authorized to do so by the California Fish and Game Commission. It is still a legal bait minnow. While the Capay Diversion Dam is a barrier to upstream movement of these fish, it is likely that red shiner will appear above the dam, given a boost by anglers releasing bait.  It will then become one more threat to the fascinating native fish community of Cache Creek.

Update by Bob Schneider: A few years ago, the district discovered that the Capay Diversion Dam was being actively undermined as flood flows poured over the dam.  It was alarming!  In a large flood event, a key part of the district water delivery infrastructure literally could fall over.

The district undertook a series of studies to determine the extent of the problem and how best to renovate the dam. Last fall, that project was completed. You can see the repair work on the district’s web site in a one-minute time-lapse video. Take a look.

The Capay Diversion Dam is an underappreciated structure in Yolo County. This small dam diverts water that supplies our farms, recharges our ground water and maintains riparian habitat. It is a gravity system that needs no pumping. In the long term, particularly in light of climate change, this will become even more important.

In Yolo County, farms are a very large part of our visual open space. A reliable water delivery system is vital to maintaining our farms and that open space.

The board and staff at the district did a great job on this project. We thank them.

— Tuleyome a regional conservation organization based in Woodland. Peter Moyle is a fisheries expert at UC Davis and Bob Schneider is the senior policy director for Tuleyome. This column appears monthly.

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The Capay Diversion Dam across Cache Creek is a simple concrete structure, 474 feet long and 15 feet high, built in 1914 and showing some wear and tear with age. In 1994, it was modernized with the addition of an inflatable dam, billed as the “longest single bladder dam in the world,” on top of the old concrete. Courtesy photo

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