Dennett Dam Removal

Update Status: Cost update 3/7/2016

Project Lead: Tuolumne River Trust

Potential Project Partners:
No partners identified at this time

Short Project Description:
Removal of Dennett Dam, an abandoned low-head dam located on the lower Tuolumne River in Modesto, California. The dam has been an instream barrier to anadromous fish passage, controlling local hydraulic and sediment transport conditions, for over 60 years, while also impeding water flow in the river. It is also a significant safety hazard adjacent to a major park, and has been the location of three drowning deaths in the last five years, including two children.

Long Project Description:
The specific short-term goal of the project is to remove Dennett Dam and restore the channel and adjacent riparian vegetation. By removing the dam, we expect to achieve the following specific long-term objectives:

  • Improve upstream passage for anadromous fish
  • Improve downstream rearing habitat and passage for juvenile anadromous fish
  • Reduce non-native predatory fish habitat
  • Improve riparian and shaded riverine aquatic habitat at the project site
  • Remove an impediment to water flow within the river channel

The project also will achieve two key community objectives:

  • Remove a hazardous in-stream structure to reduce the risk of drowning in the river
  • Improve recreational boating opportunities through the project reach

With the removal of Dennett Dam and associated channel and riparian restoration, we expect several important ecological features and functions to be rehabilitated at the project site. Most notably, we expect fish passage to improve noticeably providing unimpeded access for anadromous fish to 37 miles of the lower Tuolumne River, including access to the prime spawning grounds for steelhead trout and fall run Chinook salmon. An assessment of fish passage at the dam completed for the Tuolumne River Trust by HDR Engineering, Inc. based on NMFS and CDFW protocols of fish passage, and using hydraulic and survey data analyzed with HEC-RAS v.4.1, determined that Dennett Dam is a partial barrier that does not meet the selected fish passage criteria for a 1-foot hydraulic differential across the dam crest for flows less than 1,800 cfs. The analysis also developed a flow-frequency relationship, which estimates a 1,640 cfs flow has an exceedance probability, during adult salmon migration, of 20%. To be more explicit, approximately 80% of the time, the dam creates a passage barrier to up migrating adult salmon. By removing Dennett Dam, we remove this passage barrier and improve passage in general.

We also anticipate that downstream passage and rearing habitat for juvenile salmon and steelhead will improve. As noted above, there are a number of exotic fishes that inhabit the Tuolumne River, including black bass and striped bass, which are known predators of juvenile salmon. According to a Sediment Management Proposal prepared for the Tuolumne River Trust by NewFields River Basins Services, LLC, Dennett Dam creates a backwater effect extending approximately 4.36 miles upstream. This slow moving water behind the dam creates excellent warm water habitat for these predatory fish. On the downstream side of the dam, two large eddies on either side of the river also create very good habitat for predator species to lie in wait as juveniles flush over the middle section of the dam. Dam removal will restore sediment transport, create a more natural channel with a defined thalweg and associated pools, thus reducing predator habitat.

In addition, dam removal will reduce solar heating of the river water, thus reducing temperature stressors on the fish, particularly juveniles. By removing the slow, shallow pool in the river, water will have less of a chance to heat in the air. After removal of the dam is completed, we will undertake riparian restoration in the immediate project vicinity, which will in turn, provide more shaded riverine aquatic habitat providing more shading of the river water itself. Simultaneously reducing predator habitat and increasing shaded riverine aquatic habitat will reduce stressors on juvenile salmonids and have the combined effect of improving downstream migratory and rearing conditions for the fish.

Unique Project Characteristics:
This project can be completed in conjunction with other flood damage reduction, parks development, and habitat restoration projects, such as the development of the Tuolumne River Regional Park, the replacement of the 7th Street Bridge, or other projects in the vicinity.

Project Status Implementation
Project Cost $600,000
Project Timeframe 1-5 years
Cost-sharing US Fish and Wildlife Service contributed $105,000 and City of Modesto contributed $10,000 towards completing the Basis of Design Report
Multi-benefit Project Yes
Types of benefits The project would improve flood risk management (reduced loss of life), promote ecosystem functions, and improve recreation, making it a multi-benefit project.
Source of Project Tuolumne River Trust

Background Information:
None provided

Latitude:  37.627498      

Longitude: -120.987532