1. Reservoir Sedimentation and Wildfire in the Sierra Nevada Mountains, CA
J. Toby Minear and Scott Wright
California Water Science Center
United States Geological Survey

2. Outline
Introduction
What is reservoir sedimentation?
How does fire affect sedimentation?
‘3W’ model for reservoir sedimentation in California
Sedimentation study with Sierra Nevada Conservancy
How we could use your help…

3. Reservoir Sedimentation
Reservoir blocks downstream transport of sediment
Sediment accumulates in the reservoir = reservoir sedimentation
Reduced sediment supply to downstream areas
“Hungry water” = downstream erosion of bed and banks
Sediment
Erosion of bed and
banks from “Hungry water”
Dam
Reservoir
sedimentation
Sediment
Water

4. Effects of Fire on Sediment Dynamics
Fires in S. CA and Coast Ranges:
Fires can increase sediment loads times the pre-fire level
In particular, the ‘fire-flood’ scenario
leads to highest sediment rates
Typically 5+ years before rates return
to pre-fire levels
Effects are not as well
known for the Sierras
CalFire fire history database

5. Modeling Reservoir Sedimentation in the Sierras
Mutiple dams in the same watershed and changes to trap efficiency with time are major issues
Minear and Kondolf (2009) came up with a method to address this issue: ‘3W’ model
Estimates long-term sediment yields from reservoir sedimentation records; applies these yields to unmeasured reservoirs
Accounts for multiple dams in the same watershed and changes in trap efficiency
User-chosen time-step (yearly time-step for 2009 paper) and number of sub-regions (geomorphic regions for 2009 paper)

6. Reservoir Sedimentation in California: ‘3W’ Model
- 12 geomorphic
regions
- 1,391 dams
- 70 dams with measured
sedimentation rates
3W model: statistical
approach; based on
geomorphic region
to estimate median
sediment yield
Black dots = measured reservoirs
White dots = unmeasured reservoirs
Minear and Kondolf, 2009, WRR

7. 3W Model Results Sediment yield rates by geomorphic region:
Minear and Kondolf, 2009, WRR

8. 3W Model Results
Estimated reservoir capacity remaining in 2008 (as percent of original)
Minear and Kondolf, 2009, WRR

9. Estimated reservoir sedimentation in acre-feet
3W Model Results
Estimated reservoir sedimentation in acre-feet
Minear and Kondolf, 2009, WRR

10. Limitations of the 3W model
2. Difficult to include GIS attributes / modeling
- e.g watershed parameters, fires, soils, climate,
climate change, etc.
Reservoir B,
Bulit 1957
Reservoir D,
Bulit 1984
Reservoir C,
Bulit 1933
Reservoir E,
Bulit 1964
3. No hydrology
Reservoir A,
Bulit 1949
hydrology
1. Not GIS friendly
Reservoir F,
Bulit 1947
time

11. Objectives of this study
1. Determine better reservoir sedimentation rates in the Sierra, particularly related to fire 2. Compile existing reservoir sedimentation records into a single, publicly accessible database **We could use help from local partners to help obtain the sedimentation and capacity data

12. Improved Reservoir Sedimentation Model
Addressing sediment concerns and long-term reservoir storage in the Sierras (both in the reservoir and downstream) + +
Dam data
(size, date constructed,
sedimentation, operation)
GIS data: watershed factors
(fire history, slope, aspect, watershed size, mainstem length, etc.)
Gage data
(hydrology + sediment)

13. Publicly Accessible Database
The RESSED database:
Interagency project to update nation-wide reservoir
sedimentation database:

14. Future Steps Next Phase: Field studies of individual reservoirs
Determine sedimentation rates related to fire
Additional bathymetric mapping, coring, sediment mapping
Will depend on finding interested partners

15. Questions?