Sun River Fish Populations
Photo of the Sun River Canyon below Diversion Dam
This is a great question, and there has been much debate about this over the years. Some folks say it is because of drought. We do not believe that is true because trout populations have remained fairly high in both forks of the river above Gibson Reservoir in spite of drought. Also, FWP surveys back to the 1970's indicate mainstem trout populations are consistently low.
Others say that the huge 1964 flood ruined the habitat in the mainstem Sun. There is no doubt “the flood” changed the river channel. However, these changes occurred throughout the system and one can still see the effects of the big flood even in the pristine areas above Gibson Reservoir. Fish populations continue to thrive up there in spite of the disturbance.
Some other folks say that the fish population in the mainstem is naturally low because the river used to dry up naturally before any irrigation development occurred. While this is a fairly common belief, there is no information to support it. The river never dries up upstream from Gibson Reservoir and never really comes close to that. In dry years, the river flow can get extremely low in some areas, but this only occurs below major diversions where water is removed for irrigation. “Hydrologists” (experts who study river flows) agree there is no evidence the Sun River commonly dried up in its natural state.
Another possible explanation for low river trout populations is there are few good tributary streams for trout to spawn in. While tributaries are nice to have, they are not essential to have good river trout fisheries. Trout are very capable of spawning and rearing in big rivers. There are some top notch rivers in Montana, like the Bighorn and Madison, that have few spawning tributaries yet have excellent trout populations.
We believe the low fish populations in the mainstem are caused primarily by man's activities. The development of the extensive irrigation system in the drainage has profoundly affected the biological system. When the Sun River area was settled, legal rights were granted to use substantial amounts of water for irrigation. This resulted in the construction of an extensive set of diversions, reservoirs and canals to deliver water to agricultural lands. This development dramatically increased agricultural production, mostly of wheat, barley, hay and alfalfa. However, the development of the extensive irrigation system meant there would not be enough water left in the river for healthy fish populations.
Besides affecting river flow, the operation of the irrigation system causes some other problems. When the river flow gets too low, the water heats up faster. Because of low flows, summer water temperatures are often too warm for healthy trout in the mainstem Sun River.
There are many “diversion structures” on the river that force water into irrigation canals. None of these have fish screens on them, and we know substantial numbers of river fish are lost in these canals when they are shut off in the fall at the end of irrigation season. Also, in some areas like Muddy Creek, too much irrigation wastewater returns, causing major amounts of bank erosion. This makes the water too muddy for trout to live in. In fact, the lower 18 miles of the Sun River, immediately downstream from Vaughn where Muddy Creek enters, is now virtually uninhabitable by trout because of extremely high silt levels.
Confluence of Muddy Creek and the Sun River, showing extremely high sediment contribution from Muddy Creek
