Simulated Beaver Structures in Rocky Mountain National Park

A simulated beaver structure ten days after installation, with water pooling behind it. NPS/C. Turner.

In September 2019, Rocky Mountain National Park built simulated beaver structures (SBSs, also called beaver dam analogues) on two of its streams. Built by people, they do what a beaver dam does: slow the water, form pools behind them and let more sediment settle in those pools. The aim is to make streams more complex, restore wetlands and raise biodiversity along their banks. They are part of the park's adaptive approach to habitat restoration under its Elk and Vegetation Management Plan (EVMP).

How they are built

The park used untreated lumber. A trench was dug across the channel about two feet past each bank and about six inches below the stream bed. Six-by-six-inch boards were cut to size, stacked and screwed together, then held in place with the dirt and rock dug from the trench. Each site has two structures, about 25 yards apart, and shallow notches as wide as the stream, cut into the top board, steer the flow to the middle of the channel so it does not break out beyond it. The structures are not meant to be permanent and will be evaluated over time.

Why build them

The EVMP's 5-year review found that willows inside the fences keeping elk out had responded unevenly. With elk browsing greatly reduced, the lack of recovery in some places was worrying, and dry conditions, especially low water tables, seemed the most likely reason. Beavers are "ecosystem engineers": their dams raise local water tables, slow the flow, trap sediment in channels and floodplains, spread floodwater over the banks and let channels shift, all of which builds wetland habitat. The park expects the structures to do the same. They have worked elsewhere, including in Yellowstone National Park and at Cheley Camp outside Estes Park.

Why not bring back beavers?

The park's wetlands need beavers, which were once plentiful there, and reintroducing them is one of many options in the EVMP. But there are no plans to do it now: beavers would add pressure on the willow and aspen the park is trying to restore. Beavers have passed through the target areas without staying, suggesting the habitat is not yet good enough. As the wetlands recover, the park expects them to return on their own, and the structures may encourage them to stay.

Checking that they work

Monitoring began in the summer of 2018, before construction:

• data loggers record stream depth, the water table and its changes, overbank flooding and flow direction;
• visiting scientists measure sediment on the floodplain and survey plants in set plots;
• sound recorders measure biodiversity.

It continues after construction, to see whether hydrology, willow regrowth and biodiversity improve. A third location, on a stream with more flow, has been proposed for fall 2020, depending on how the first four structures do; if they succeed, more may follow.

Who is involved

Staff of the park's Resource Stewardship Division, NPS Inventory and Monitoring staff, and researchers from Colorado State University specialising in wetland ecology, stream ecology and habitat restoration.

Sources

• Carissa Turner, Continental Divide Research Learning Center, "Simulated Beaver Structures," National Park Service, October 3, 2019. https://www.nps.gov/articles/simulated-beaver-structures.htm