From DNR reports

The Department of Natural Resources today announced the final results of a long-term regional study to assess how well sediment traps work to improve fish populations in rivers throughout the state.

A sediment trap is an artificially excavated pool that allows excess sand to settle out, helping re-expose downstream gravel deposits that were previously covered. Traps have been added to many streams where fish spawning habitat was negatively affected by excessive inputs and transport of sand. Use of traps to re-expose spawning gravels has been very popular in Michigan, with hundreds of traps becoming operational in the 1990s.

The siting, installation and maintenance of a sediment trap is a complicated, costly process, and one that may extend for many years, but little evidence had been collected to prove their value. DNR fisheries biologists and managers were concerned these actions were not improving habitat as intended and could perhaps even disrupt normal stream channel-shaping processes and increase streambank erosion.

The DNR’s Fisheries Division recently completed a regional survey of channel conditions upstream and downstream of 65 currently or recently operating traps.

The results of the regional survey, led by fisheries research biologists Troy Zorn and Todd Wills, were recently published in the North American Journal of Fisheries Management and showed that typical sediment trap operations in Michigan did not expose gravel, decrease sand or increase stream depth as they were intended to.

“There are a number of factors that might explain why most traps didn’t work, including local geology, trap placement in the stream or trap dimensions,” said Zorn. “In many cases, the number of traps installed and frequency of trap clean-outs may simply be too low to noticeably change stream habitat conditions.”

“On the bright side, we generally found no adverse effects of sediment traps on channel stability, but did note a couple situations where installation of traps might have contributed to channel changes,” Zorn continued.

Zorn’s and Wills’ results didn’t support the use of sediment traps as a stand-alone habitat rehabilitation tool, but they mentioned specific situations where traps may be useful. For example, sediment traps have been effective in capturing and preventing downstream transport of large, localized inputs of sand, such as those associated with dam removal. Their findings and previous assessments of sediment traps suggest resource managers should carefully consider watershed-level inputs, the river and all potential management options to determine which provide the best return on investment.