Watershed History

1. Ross Valley Watershed San Anselmo/Corte Madera Creek is the primary drainage channel for the 28 square mile Ross Valley Watershed. Download map (.pdf, 220kb)
2. Average Annual Rainfall The Ross Valley Watershed receives some of the highest annual rainfall amounts in the Bay Area. Download map (.pdf, 240kb)
3. (a) Five Day Hydrograph & Rain Seven inches of rain fell on the Ross Valley Watershed in the two weeks prior to the flood. Because of this near-saturated condition, the earth had little or no capacity to absorb stormwater on December 31, 2005. In the Ross Valley, creeks rise very quickly in both major and normal storms. Download map (.pdf, 35kb)
3. (b) December 2005 Hydrograph & Rainfall At 2:00 am on December 31, 2005, Corte Madera Creek was already at flood stage of 19 feet. The strongest part of the storm occurred from 3:00 am to 6:00 am. Flood waters topped the banks at Ross and San Anselmo at the same time The flood crested at 22.5 feet, 3.5 feet above flood stage at Ross. Download map (.pdf, 30kb)
4. Alluvial Soil Alluvial soils consist of sediment carried and deposited by flowing water over time. This map shows the historic flood flows on the valley floor - everywhere there is alluvial soil. In the Ross Valley, these areas are highly developed. Download map (.pdf, 350kb)
5. Reach Map Each of the six reaches have unique conditions that both support and restrict high creek flows. Download map (.pdf, 250kb)
6. Major Creek Constrictions In its preliminary findings, the Technical Working Group (TWG) has identified over a dozen major constrictions in the creek system that cause out-of-channel flows. In most cases, each constriction sends backflow that exacerbates the already reduced capacity upstream. Download map (.pdf, 280kb)
7. Flood Path and Depth (cities) Major flood flows begin upstream, but downstream constrictions force water out of the main channel. Download map (.pdf, 460kb)
7. Flood Path and Depth (soils) The historic flood plain, indicated by the tan color alluvial soil, far exceeds the extent of the December 31, 2005 flood, indicated by the red flooded parcels. Download map (.pdf, 455kb)
8. Reach 6 - Fairfax The culvert below the Fairfax fire house and city hall is the primary cause of flooding in Fairfax. Download map (.pdf, 715kb)
9. Reach 5 - Sleepy Hollow Creek The bridges along Sleepy Hollow Creek restrict the flow down to San Anselmo Creek. This reduced capacity is exacerbated by the constrictions downstream, which back the water up like a traffic jam. Download map (.pdf, 740kb)
10. Reach 4 - San Anselmo Downtown San Anselmo has six major constrictions, four of which are within 500 feet of each other. These severe constraints caused extensive flooding in San Anselmo and downstream in Ross, Kentfield and Larkspur. Download map (.pdf, 815kb)
11. Reach 3 - Ross Creek Ross Creek flows out of Phoenix Lake and joins Corte Madera Creek. It has been estimated to carry up to 1000cfs. The bridge constrictions at Shady Lane forced water out of the channel and increased the flood flow coming from San Anselmo. Download map (.pdf, 795kb)
12. Reach 2 - Concrete Channel Designed in the 1960s, the concrete channel conveys approximately 3500cfs. In the December 31, 2005 storm, there was no reported overflow of the concrete channel. In fact, water from Kent Avenue flowed over College of Marin property and back into the concrete channel. Download map (.pdf, 755kb)
13. Reach 1 - Earthen Channel The eathern channel was designed by the Army Corps of Engineers to convey 9600cfs. During the December 31 storm, this channel carried both normal channel flow and overland flow. Sedimentation over the last 15 years has decreased the capacity to an unknown amount. This section requires regular dredging, according to the Army Corps Interim Operation & Maintenance Manual. Download map (.pdf, 750kb) Download key (.pdf, 135kb)