Snake River Tributary Basins
Table of Contents
The Importance of Tributary Basins to the Snake River
Snake River Basin - Tributary Basin Relationships
Land Use Data
Water Level Measurements
Stream Flow Data



Basins tributary to the Snake River in the SR3 study area cover parts of Idaho, Wyoming, Utah, Nevada, and Oregon. The Snake River from Jackson Lake, Wyoming to Brownlee Dam, on the Idaho-Oregon border, is nearly 700 miles long. The tributary basins, combined with the east and west parts of the Snake River Plain, drain more than 72,000 square miles (46,000,000 acres). Depending on the scale or detail with which the Snake River drainage system is examined, there are at least 60 separate streams or drainages that connect to the main stem of the river or at least deliver water to the Snake River Plain (Kjelstrom, 1986; Peterson, 1988).

Jackson Lake is the beginning of the Snake River. The lake is nestled at the base of the Teton Mountains within Grand Teton National Park  About 15,300 square miles of the study area is associated with the Snake River Plain with an additional 56,000 square miles within the hydrologic units that extend up the tributary basins.

Water in the Snake River begins its journey flowing south through Jackson Hole, as shown in photo to left..  The major tributaries in this region include the Gros Ventre River, Hoback River, Greys River, and the Salt River.

More than twenty tributary streams feed into the Snake River in the eastern portion of the Snake River basin. The larger tributaries in this area include: Henrys Fork, Teton River, Falls River, Big Wood River, Blackfoot River, Big Lost River, and Portneuf River. The Snake River drops in elevation from more than 5,015 feet on the eastern side of the plain, to about 2,100 feet on the western edge near Weiser, Idaho.

The most downstream reach for this study, the Middle Snake-Boise sub basin, extends from King Hill to Brownlee Dam and includes the western Snake River Plain. The western Snake River Plain is about 50 miles wide and 145 miles long (Newton, 1991). The major tributaries in this region include: Payette River, Boise River, Weiser River, Owhyee River, Bruneau River, Malheur River, Powder River, and Burnt River.

The Importance of Tributary Basins to the Snake River

While the flow of water within the Snake River receives a tremendous amount of attention, it is necessary to take a closer look at the areas where the flow originates. Precipitation is the sole source of recharge in tributary basins. The Oregon Climate Service (Daly, 1998) created an average annual precipitation map for the Western United States covering the time period 1961-1990 (see map to left). This map shows that within the Snake River Plain precipitation is less than 15 inches per year with some areas receiving less than 10 inches per year. Precipitation totals in the tributary basins can exceed 60 inches per year. These maps were created using the Parameter-Elevation Regressions on Independent Slopes Model (PRISM).

Two factors stand out as reasons to include tributary basins when discussing the flow of water within the Snake River. First, the majority of recharge occurs within the tributary basins. Second, the large areas associated with the tributary basins provide opportunities for development and use of water within the tributary basins themselves. Use of water within a tributary basin can affect the quantity and timing of water availability to the Snake River.

Snake River Basin: River-Tributary Basin Relationships

A basin tributary to the Snake River represents a geographical area with a stream or other body of water, surface or underground, that contributes water to the Snake River, even in small or intermittent quantities. With so many streams and rivers that deliver water to the Snake River, the scale or size of a basin has to be considered in allocating resources for studies or management programs.

Much of the data presented here will use the term “HUCs” (hydrologic unit code), often used synonymously with cataloging units.  Cataloging units are the smallest division set aside by the USGS as a system of nomenclature dividing the United States into several large geographic basins or regions that represent hydrologic systems.  A cataloging unit may represent a portion of a basin (example- Little Lost), or a combination of drainage basins (example, Upper Snake).  In the SR3 study area, there are 53 cataloging units;  26 in the Upper Snake sub basin and 27 in the Middle Snake Sub basin.  See illustrations that follow.


Many different hydrologic relationships could exist between a river and its tributaries. Within the Snake River Basin, three main categories are readily identified:

A.  A tributary stream that is not connected to the Snake River.
B.  A tributary stream that is connected to the Snake River. Surface water and ground water are not hydraulically interconnected.
C.  A tributary stream connected to the Snake River. Surface water and ground water are at least partially interconnected.

Category A-Examples In the northeastern portion of the Snake River Plain, located within the State of Idaho, there are several streams which the Bureau of Reclamation grouped in their SR3 Blueprint (1996) under the heading “Streams that disappear into the Snake Plain Aquifer” (see figure below). These streams, located on the northern portion of the Upper Snake sub basin, are not tiny ephemeral streams. The largest of the group, the Big Lost River, annually delivers more than 200,000 acre-feet of water that travels down the river to “disappear” or infiltrate into the Snake River Plain. Others in this group include Camas-Beaver Creek, Medicine Lodge Creek, Birch Creek, and Little Lost River. Combined, these basins are the source for over 9 percent (945,000 ac-ft) of the annual water contributions to the Upper Snake River Basin (data from Kjelstrom, 1986).

Category B-Examples: IDWR (Idaho Department of Water Resources, 1997) has described the Rexburg Bench- Moody Creek region as an area that is not hydraulically interconnected with the regional aquifer. In another example, Horn and Jeong (1989), have suggested a similar interpretation describing the Camas Creek basin, Idaho.

Category C-Examples: The third category represents most of the tributary basins. Most of the tributary streams are considered to have at least some degree of interconnectedness with their associated aquifers. It may be difficult in some areas to determine if the associated ground water is the regional system or a perched zone.

Land Use Data
 Recent land use GIS data were obtained from the respective state agencies of Idaho, Wyoming, and Oregon and used to identify presently irrigated lands (see figure below). Oregon is in the process of converting older data sets to useable GIS files; however, they did provide that portion of the data currently available. There are more areas of agricultural use than shown in Oregon and that data should be available to incorporate into a data set in the near future. Data from Idaho did enable acreage tabulation of the irrigated and non-irrigated lands. See the table below which provides a summary of the distribution of irrigated and non-irrigated acres for HUCs in Idaho.

Teton, Raft, Portneuf, and Blackfoot are those HUCs which contain significant non-irrigated lands with potential for conversion to irrigation. Of the eight HUCs with the largest irrigated acreage, seven are located primarily on the Snake River Plain, see figure below.

Water Level Measurements

The water table data from monitoring wells, provided by the USGS, contains measurements from more that 1300 wells within that portion of the Snake River Basin located in Idaho (see figure below). A summary of the information obtained from these wells is published by the U.S. Geological Survey in cooperation with the State of Idaho and other agencies in their annual report entitled Water Resources Data-Idaho. The majority of wells are located within those regions where competition for water has been more intense, such as the Eastern Snake River Plain.

Most wells show annual fluctuations of less than ten feet. Notable exceptions include wells in Bruneau, Willow, and Teton where annual fluctuations can vary to a maximum of 50 to 65 feet .

The figures below summarize the water table data.  The first figure list locations of wells used to illustrate changes in ground water levels.  The second figure illustrates the fluctuations in the water table over time.

Figure 1.

Figure 2.

Stream Flow Data

Most tributary streams are considered to be at least somewhat hydraulically interconnected with the aquifer, changes in stream flow can affect the amount of water available for aquifer recharge. Also, significant changes in the quantity of ground water diverted or the amount of surface water consumptively used might be indicated by a reduction in stream flow relative to other nearby tributaries.   See following table for a summary of stream flows in the Snake River Basin.

For a more detailed look at the SR3 Tributary Basins see the Research Technical Completion report, "Assessment of Needs and Approaches for Evaluating Ground Water and Surface Water Interactions for Hydrologic Units in the Snake River Basin", 1999, Lovell, Mark D. and Johnson, Gary S., Idaho Water Resources Research Institute, University of Idaho, Moscow, Idaho, p. 170.  For the references used for this page click on references below: