Assessing Groundwater/Surface Water Interaction and Stream Discharge Uncertainty in the Snake River

Science Center Objects

The State of Idaho needs to determine stream discharge, adjusted to remove fluctuations resulting from the operation of hydropower facilities, in the Snake River below Swan Falls Dam near Murphy, Idaho. The State will use this information to distribute water to owners of water rights in the middle Snake River, particularly at thresholds of 3,900 and 5,600 ft3/s.

Water exchange between surface water and groundwater can play a role in the calculation of “adjusted” discharge and the availability of water for water rights owners, but that exchange is not easily quantified.

Additionally, all discharge data have some level of uncertainty, and that uncertainty should be considered for the State of Idaho to effectively monitor and manage water rights.

We quantified the amount of water exchanged between groundwater and surface water in the Snake River from King Hill to Murphy, Idaho, in November 2012 and July 2013.

We also estimated the uncertainty in measured and computed discharge at Idaho Power Company streamgages on the Snake River downstream of Lower Salmon Falls Dam to downstream of Swan Falls Dam.

The State of Idaho will use this information to develop a protocol to calculate and report an adjusted discharge needed to manage hydropower and minimum discharge water rights in the middle Snake River.

  • In November 2012, we measured a gain in stream discharge of 415 ft3/s in the river reach from King Hill to CJ Strike Dam; 217 ft3/s was gained in the river reach from the Idaho State Highway 51 bridge to CJ Strike Dam. Groundwater inflow to the river is the source of the gains in stream discharge.
  • No significant gains from groundwater to surface water were measured from CJ Strike Dam to Murphy in either the November 2012 or the July 2013 survey.
  • Uncertainty in computed discharge at the Idaho Power Company streamgage in the Snake River below Swan Falls Dam near Murphy was 10.1 and 6.0 percent at thresholds of 3,900 and 5,600 ft3/s, respectively. So, a reported discharge of 3,900 ft3/s may actually be anywhere between 3,500 ft3/s and 4,300 ft3/s.

All streamgage and discharge measurements have some level of uncertainty that cannot be entirely eliminated. In the future, uncertainty might be reduced by selecting better measurement and streamgaging sites.