Mill Creek Fishery
Mill Creek is one of the Sacramento River's major tributaries and one of only three watersheds in California's Central Valley that support self-sustaining, genetically isolated runs of wild Spring Run Chinook Salmon, a state and federally designated endangered species, as well as Fall Run Chinook Salmon and Central Valley Steelhead Trout (also endangered).
Mill Creek is essentially undammed, with only three man-made in-creek structures consisting of low irrigation diversions operated by the Los Molinos Mutual Water Company. They are designed to enable migrating fish to swim over them in high water, and also have well designed fish ladders for low water conditions. Combined with the virtual lack of development above the Valley floor, Mill Creek thereby provides essentially pristine spawning and rearing habitat for the great majority of its length.
Spring Run Chinook Salmon migrate up the Sacramento River from the Pacific Ocean to enter Mill Creek mainly in the months of April, May and June, during the spring runoff from melting snow. Once in the creek these fish immediately swim far upstream in search of deep, undisturbed holding pools where water temperatures remain cold throughout the summer months.
Mill Creek's Spring Run salmon are unique in that they are known to spawn as high as 5,000 feet above sea level, the highest altitude known for North American salmon. Spawning occurs from Late August to mid-October. Unlike most young Chinook salmon, which begin their journey to the ocean within a few months of hatching, juvenile Spring Run fish generally remain in Mill Creek for six to eighteen months.
Water temperatures in Mill Creek below 1,000 feet are too warm in summer months to sustain adult Chinook Salmon, thereby creating an impenetrable thermal barrier ensuring wide separation of spawning Spring and Fall Run Chinook Salmon, thus protecting and preserving the unique genetic traits of the Spring Run species.
Fall Run Chinook Salmon enter the mouth of Mill Creek from late October through November, taking advantage of higher stream flows and cooler water caused by the first autumn rains. They then spawn almost immediately, primarily within the creek's valley floor reach. Juveniles hatch from December through March and migrate to the ocean the following spring.
Steelhead Trout return to Mill Creek from the ocean primarily in late fall and winter, during the high flows from winter storms. Like Spring Run Salmon, they migrate far upstream to spawn. Spawning soon after arriving, their young hatch the following spring and typically remain in the creek for one to two years before migrating to the ocean.
Populations of all three anadromous species have declined significantly from historic averages, especially within the past 30 to 40 years. However, Mill Creek's habitat and rearing conditions have remained essentially pristine during that period and survival rates of life stages occurring within Mill Creek (holding and spawning adults and rearing juveniles) have not measurably diminished since salmon counts were first made. It therefore seems clear that the decline in numbers of Mill Creek's Chinook Salmon and Steelhead Trout are the result of degraded environmental conditions in the mainstem Sacramento River, the Sacramento/San Joaquin Delta and the ocean.