River Stage and Bottom Elevation ESHMC Meeting 6 May 2008 Stacey Taylor

Overview Model river representation in ESPAM 1 What data is available to make changes for the new stress periods (1 month) Changes needed to represent the Snake River Changes needed to represent American Falls Reservoir Summarize

The Snake River in ESPAM 1 MODFLOW river package Terms include K of riverbed sediments, width of river, length of river in model cell, elevation of river, thickness of material below the river (river bottom sediments thickness) 6 month stress periods River above Milner Dam is represented using MODFLOW river cells Stage elevations based on digitizing the Snake River using 10m DEMs Linearly interpolated between river depth of known points along the Snake River (based on cross-sections) Assumed 30 ft river bottom sediments thickness American Falls Reservoir elevation based on topographic maps

Head in aquifer (Haq) in hydraulic connection with river MODFLOW River Package Head in aquifer (Haq) in hydraulic connection with river McDonald and Harbaugh, 1988

MODFLOW River Package (cont) Aquifer head (Haq) loses hydraulic contact with river McDonald and Harbaugh, 1988

River Cells in ESPAM 1 Ashton Rexburg Heise Shelley Near Blackfoot Neeley Minidoka Legend USGS gage River cell

American Falls Reservoir in ESPAM 1

What’s the Problem? We need to adjust for the 1-month stress periods This means that average river stage elevations and American Falls Reservoir elevations must also be adjusted.

Data Available for River Stage USGS stream gage elevation data (USGS NWIS) Gage height based on USBR telemetry (Almost) monthly data is available Time between measurements not always uniform Ex: Measurement 1 – taken on 4/22/07 Measurement 2 – taken on 5/10/07 Measurement 3 – taken on 7/1/07 USGS Gage Data Collection Initiated Comments Ashton 1920 Monthly data; some months have no measurement Rexburg 1973 Heise 1913 Shelley Blackfoot Neeley 1969 Minidoka 1963 Mostly monthly; Nov. 1963 through October 1980 lacks monthly data

Data Available for River Bottom Elevation No additional data is available for the bottom elevation for the river cells Use current linear interpolations based on cross sections between gage stations

Data Available for American Falls Reservoir Daily reservoir surface elevation and reservoir storage available from the USBR Hydromet historical data at American Falls Take averages to estimate the value for the monthly stress periods Images of American Falls reservoir for: Aug 1986 Jul 2002 Jun 1989 July 2004 Jul 1992 Jan 2005 Mar-Oct 2000 Sep 2006 No bathymetry is available for American Falls Reservoir

Changing Area of American Falls Reservoir March 2000 July 2000 October 2000

August 1986 vs. August 2000 August 1986 August 2000 *Can’t necessarily assume same cell distribution for the same months from year-to-year

How do we make the change to 1-Month Stress Periods? American Falls Reservoir stage gage Need to adjust for changing reservoir extents by changing the number of river cells representing American Falls Reservoir

Simulating STAGE in American Falls Reservoir with the River Cells Change river cells for each stress period based on available images For the years lacking images, monthly storage/stage data can be compared. Ex: ? July 2000 Storage = 100,000 ac-ft Gage @ American Falls = 4341 ft January 2006 Storage = 100,132 ac-ft Gage @ American Falls = 4340 ft

Stage elevation data available here at American Falls Simulating STAGE in American Falls Reservoir with the River Cells (cont.) Stage elevation data available here at American Falls Approximate Elevation based on topo map: 4360 ft amsl Assume linear gradient between points for stress period

American Falls Reservoir Bottom Elevation No bathymetry data is available Assume flat bottom with slope = -0.045 Assign cells bottom elevation based on the slope of the line for the bottom elevation Elev ~ 4360 ft 96,900 ft (when reservoir is full) Dam crest elevation = 4366.5 ft Snake River entering American Falls Reservoir Slope = -0.045 DAM Bottom Sediments in American Falls Reservoir

Summary Stage elevation for river cells representing the Snake River will be based on USGS gage station data Missing data for a month will be averaged based on preceding and following month Bottom elevation will remain the same American Falls Reservoir stage will be based on the American Falls gage (south end) and an assumed linear gradient to the north end American Falls Reservoir bed (bottom elevation) will be based on a linear gradient between the base of the dam and elevation at the maximum extent of the reservoir Continue to assume 30 ft bed thickness for all river cells