How Dams and Irrigation Change the Yellowstone River's Flow, 1928–2002

Major floods in 1996 and 1997 raised public concern about how human activity affects the Yellowstone River in Montana. In 1999 the Yellowstone River Conservation District Council — mostly representatives of the conservation districts along the main stem — was formed to promote wise use of the river's resources. With no major dams or reservoirs on its main stem, the Yellowstone is one of the longest free-flowing rivers in the United States, yet many uses still alter its flow. The council and the U.S. Army Corps of Engineers set out to understand the cumulative effects of water development in the basin, and the U.S. Geological Survey supplied the streamflow science.

How water use shifts the river

Since the 1800s, water in the basin has been diverted for irrigation, mining, towns and other uses, and streamflow has been measured at sites across the basin since the late 1800s. Any gauge reading reflects water taken out for irrigation, held in tributary reservoirs and used in other ways, and water put back by reservoir releases and irrigation return flows.

Irrigation systems and reservoirs generally hold or divert water in spring and summer and release it in fall and winter. So spring and summer flows are lower than they would be naturally — also because of evaporation and seepage — while fall and winter flows are higher. Besides the water that flows on into the Missouri River, some water leaves the basin every year through evapotranspiration from reservoirs and irrigated fields and seepage from reservoirs and canals.

Two versions of the river

Starting in 2010, the Corps of Engineers, the council and the USGS published four reports on streamflow at sites in the basin for 1928–2002. From observed gauge records they built daily streamflow for two scenarios: unregulated flow, as if there were no water development, and regulated flow, with it. They then calculated annual means, low flows and peak flows under both, for sites on the Yellowstone and on the Bighorn, Tongue and Powder rivers.

Through a year. In a typical water year, 1979, regulated flow at both Billings and Forsyth falls below unregulated flow in spring and summer, because of diversions for irrigation, reservoir storage and other uses.

Low flows

Low flows matter to irrigators who need enough water for crops, to towns for domestic, commercial and industrial supply, and to water quality, fish and other river life. They are described by duration — the lowest average flow over any 7 or 30 consecutive days in a year — and probability. The 7-day 50-percent low flow (the "7-day, 2-year" low flow) has a one-in-two chance each year of flows falling below it; the 20-percent low flow (the "5-year" low flow), a one-in-five chance.

Floods

Peak flows improve fish habitat, soak wetlands and help seedlings establish, but they can also reshape the channel by scouring and depositing sediment, and damage communities and infrastructure. The 50-percent flood (the "2-year flood") has an even chance of being equalled or exceeded in any year; the 20-percent flood (the "5-year flood") a one-in-five chance; and the 1-percent flood (the "100-year flood") a one-in-a-hundred chance.

• At Yellowstone Lake outlet, Corwin Springs and Livingston, regulated and unregulated floods are nearly the same: there is little development upstream.
• At Billings, unregulated peaks are slightly higher, as in the 1979 hydrograph.
• Below Billings the gap widens sharply, mainly because of the heavily regulated Bighorn River, which joins upstream of Forsyth.

Depletions

Much of the Bighorn basin's water goes to agriculture, and reservoirs were built to store it. The U.S. Bureau of Reclamation estimated depletions — changes to natural flow caused by water development — averaged over 1928–2002. Most represent water lost from the basin to evapotranspiration from reservoirs and irrigated fields and to seepage into groundwater. Development increases steadily from Livingston to Billings and jumps at the Bighorn confluence. Comparing losses from irrigation alone with those from irrigation plus storage shows that the Bighorn basin's reservoirs — Boysen Reservoir, Buffalo Bill Reservoir and Bighorn Lake — account for a large share of the water that leaves the Yellowstone basin.

Using the results

Scientists are using the regulated and unregulated statistics to understand how water development has changed the river's flow, and how that has affected floodplain extent, low-flow hydraulics, fish habitat and water quality. Water managers are developing practices to make the river healthier while balancing its many uses, and the studies give them a basis for decisions about its future.

Sources

• Eddy-Miller, C.A., and Chase, K.J., 2015, Effects of water-resource development on Yellowstone River streamflow, 1928–2002: U.S. Geological Survey Fact Sheet 2015–3004, prepared in cooperation with the Yellowstone River Conservation District Council and the U.S. Army Corps of Engineers. https://pubs.usgs.gov/publication/fs20153004
• Figures and photos from the fact sheet's PDF: U.S. Geological Survey; Yellowtail Dam, National Park Service.
• Rewritten in hubnx's own words.