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

2. 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

3. 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

4. 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

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

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

7. American Falls Reservoir in ESPAM 1

8. 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.

9. 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 through October 1980 lacks monthly data

10. 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

11. 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 Jul 2002
Jun July 2004
Jul Jan 2005
Mar-Oct Sep 2006
No bathymetry is available for American Falls Reservoir

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

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

14. 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

15. 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
American Falls = 4341 ft
January 2006
Storage = 100,132 ac-ft
American Falls = 4340 ft

16. 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: ft amsl
Assume linear gradient between points for stress period

17. American Falls Reservoir Bottom Elevation
No bathymetry data is available
Assume flat bottom with slope =
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 = ft
Snake River entering American Falls Reservoir
Slope =
DAM
Bottom Sediments in American Falls Reservoir

18. 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