Damned if You Do Dam, Damned if You...Well, Don't Dam: The Days of Dennett Dam are Done

One does not usually equate the city of Modesto with dams and reservoirs. It's true there is a Modesto Reservoir, but it is fifteen miles east of the city. But there is a dam in the city limits, and it has been a problem and an eyesore for decades. Back in 1933 someone had the bright idea of building a small dam on the Tuolumne River by the 9th Street bridge and constructing a nice park built around boating and swimming. Dennett Dam was built, and it lasted an entire seven years before being mostly destroyed by flooding. The park never materialized, and the remains of the dam lay abandoned. At some point, someone thought it would be a good idea to cut an opening in the dam to allow fish to surmount the structure and swim upstream (the Tuolumne is a salmon river).

The unintended consequences mounted. The opening set up unpredictable whirlpools in the river current, and at least three people have been trapped and drowned nearby. The structure impeded flows of the river and during low water, vast amounts of the invasive weed River Hyacinth would accumulate, blocking the migration of fish, and crowding out native vegetation. And the fish, which the opening was meant to help, were devastated by predators, especially introduced bass. The newly born fish were forced to utilize the narrow opening, and were easy marks for the predator fish waiting in the deep pools beyond.

Dennett Dam during a bad River Hyacinth year. Picture courtesy of Patrick Koepele of the Tuolumne River Trust

What was to be done? No one wanted to own the dam or be responsible for the problems it was causing. The city of Modesto set aside some funds around 2006, but unexpectedly high costs torpedoed the project. In 2010, the Tuolumne River Trust got involved and began to organizing fundraising efforts, both from individual contributions and grants from the State Lands Commission, California Department of Fish and Wildlife, California Department of Water Resources, USA Fish and Wildlife Service, and local agencies. Ultimately, $1.6 million was raised and work has begun. It's sort of a complicated process.

For one, they need to move the river out of the way, so the construction firm (Innovative Construction Solutions) has cut a temporary channel around the dam. It will be filled in when the dam is removed. The deep pools below the dam will need to be filled in.

When the project is completed, a major impediment to the migration of salmon will have been removed, and some 37 river miles will become more available for spawning. The land around the dam is slated for park improvement, although much or most of the "development" will consist of rebuilding the riparian habitat that had been disrupted by agricultural and industrial development. The river can become the treasure it has always had the potential to be.

If you want to see the progress over the next few weeks (or months?), it's easy to find. The dam is almost under the 9th Street Bridge in south Modesto. There is a pedestrian sidewalk on the bridge, so just park nearby and walk out.

Note: I stole the pun that I used in the title. Years ago my dad was working on a paper that he ended up titling "Damned if you do Dam, Damned if you don't Dam". I've always appreciated it.