Gibson Dam to Diversion Dam
Canyon area upstream of Diversion Dam on the Sun River
The mainstem Sun River flows for about two miles through a rocky canyon from Gibson Dam to Diversion Reservoir. Water flow in this short section is very high during the summer irrigation season when water is released out of Gibson Reservoir for downstream irrigation needs. At the end of the irrigation season, the flow in this section drops from a normal summer flow of 1,400-1,500 cubic feet per second to the typical fall/winter level of 50-150 cubic feet per second.
Diversion Dam to Elk Creek
Sun River in the Alkali Flats area
The first 12 miles of the Sun River below Diversion Dam is entrenched in a narrow valley about 100 yards wide. The valley then broadens out to about 400 yards wide for the next 20 miles down to the confluence of Elk Creek near the town of Augusta. Benchlands composed of shale, limestone, and glacial till border the river and rise upwards of 100 feet above the floodplain. Riparian (streamside) vegetation is sparse in this reach because of the narrow floodplain. Scattered stands of cottonwoods and willows border the river, with undergrowth of rose and some Russian olive. Where the floodplain widens out 20 miles above the mouth of Elk Creek, deciduous woodland dominated by cottonwoods comprises the riparian zone. The reach of the mainstem Sun River above the confluence of Elk Creek near Augusta is sometimes referred to as the North Fork of the Sun River.
Flows in this section of the Sun River are too low to support a healthy trout population. Montana Fish, Wildlife and Parks recommends a minimum flows of at least 220 cubic feet per second during non-drought years and an absolute minimum flow of 100 cubic feet per second during drought years in this river section. Average monthly flows below Diversion Dam have dipped below the absolute minimum flow target of 100 cubic feet per second in 7 of the last 10 years (1995-2004). On average, monthly flows were below the 100 cubic feet per second target for about three months per year during that 10-year period.
Elk Creek to Muddy Creek
Simms Electrofishing Section on the Sun River
The river occupies a fairly wide valley in the 48-mile stretch from the mouth of Elk Creek to Muddy Creek near the town of Vaughn. Here the riparian zone is a cottonwood-dominated woodland with rose and willows being common shrub species found in the understory.
This reach of the Sun River often has inadequate flows, which causes sedimentation problems, high water temperatures, and low fish populations. One of the most severely dewatered reaches in this section is near Lowry Bridge upstream from Simms. Further downstream, this reach receives recharge water and a significant amount of sediment-laden irrigation return water from irrigated lands on the north side of the Sun River.
Montana Fish, Wildlife and Parks recommends minimum flows of at least 220 cubic feet per second during non-drought years and an absolute minimum flow of 130 cubic feet per second during drought years in this reach. The U.S. Geological Survey monitors river flow in this reach with a gage at Simms, which is located about 20 miles downstream from Elk Creek and eight miles downstream from the Fort Shaw Diversion. Average monthly river flow at Simms dipped below the absolute minimum fish flow target of 130 cubic feet per second in seven of eight years during the period 1997-2004. Average daily flows of only 20-30 cubic feet per second have been measured in recent years.
Muddy Creek to Missouri River
The confluence of Muddy Creek and the Sun River, showing extremely high sediment contribution from Muddy Creek
Muddy Creek is the last major tributary to the Sun River before it joins the Missouri River in Great Falls. Turbid waters from Muddy Creek have profound impacts on the lower 18 miles of the Sun. Muddy Creek flows about 40 miles in a southeastly direction to where it joins the Sun River near the town of Vaughn. The Muddy Creek drainage borders the Teton River, Freezeout Lake, and the Benton Lake drainages and covers an area of 314 square miles. Muddy Creek was once an intermittent prairie stream with a broad floodplain.
Large amounts of irrigation wastewater (surface and subsurface) from about 60% of the 84,000-acre Greenfields irrigation project drain into Muddy Creek. This has dramatically altered streamflow and channel conditions on the creek. Summertime flows in Muddy Creek now average about 240-280 cubic feet per second and base flows (during winter) average 30-50 cubic feet per second. These flows are 10-20 times higher than natural for Muddy Creek. Summertime flows in Muddy Creek are now often far higher than in many sections of the Sun River itself.
These unnaturally high flows have caused the channel of Muddy Creek to downcut through silty soils and become highly entrenched. The degree of channel downcutting increases in a downstream direction. On the lower end of the drainage, the stream channel has eroded 30 feet vertically, or more. This downcutting causes severe bank erosion and results in very high sediment loads to the Sun River.
Below the mouth of Muddy Creek, the Sun River channel is entrenched and confined by sedimentary benchland deposits. On about the lower four miles of the river, the floodplain is severely constricted by dikes built to prevent flooding on the west side of the City of Great Falls. The channel characteristics and flow of the lower river are also influenced by the impoundment of water by Black Eagle Dam, located on the Missouri River approximately four miles below the Sun River confluence. The large, man-caused sediment load from Muddy Creek has severely impacted the fishery and aquatic community in this area. It also dramatically affects the habitat and channel stability of Muddy Creek, the Sun River, and even in the Missouri River between the mouth of the Sun River and Morony Dam. In recent years, irrigation discharge reductions and channel stabilization work in the Muddy Creek drainage have substantially reduced erosion, but sediment levels and flows are still very high.
