Dry Creek Watershed Detention Reconnaissance Study

Updated on 2/4/2022: Short Project Description and Long Project Description

Project Lead: Stanislaus County and City of Modesto

Potential Project Partners:
USACE

Short Project Description:
Complete a reconnaissance study of potential options for reducing flood risks by detaining flood flows in the Dry Creek watershed, upstream of the City of Modesto.

Long Project Description:
While the main purpose of this project is to reduce flood risk for lands in and around the City of Modesto, it would also have the effect of reducing flood flows entering the Tuolumne River and thence the San Joaquin River, as well as the potential for recharge of local groundwater aquifers. To our knowledge, the effect of a hypothetical detention facility in the Dry Creek basin has not been quantitatively evaluated.

According to the current FEMA Flood Insurance Study, the 100-year flow on Dry Creek at Modesto (192.3 sq.mi.) is 11,800 cfs; whereas the 100-year peak flow on the Tuolumne River at Modesto (1884 sq.mi.) is 70,000 cfs. In a 10-year event, the values are 4,730 cfs and 10,500 cfs, respectively. The much smaller Dry Creek watershed would no doubt peak prior to the Tuolumne River watershed, which is regulated by the New Don Pedro reservoir. As a back-of-the-envelope assessment, a 30,000 AF storage facility could contain 24 hours of an average 15,000 cfs flow. This suggests that such a facility would have the capacity to detain most or all of Dry Creek’s flood flows in a large event, perhaps creating a 2,000-8,000 cfs reduction in Dry Creek contributions to the Tuolumne River and on the San Joaquin River at the time of peak flows in those larger systems. This opportunity to reduce inflows to the San Joaquin River may be similar to, or larger than, the flow reduction resulting from managed detention on the San Joaquin River at the National Wildlife Refuge, which was evaluated at approximately 3,000 cfs in a 35,000 cfs event on the San Joaquin River (ESA, 2015).

Unique Project Characteristics:
Dry Creek in Stanislaus County has the largest uncontrolled flow in the San Joaquin River basin, which affects both downstream and upstream flood levels within the system.

Background Information:
None provided