Channel Morphology Response to Selective Wood Removals in a Sand-Laden Wisconsin Trout Stream

Abstract

Large sand bed loads in trout stream headwaters can limit salmonid spawning habitat and reproductive success. This phenomenon has been observed in many northern Wisconsin watersheds, where historic logging practices are the likely source of the sediment loading. Presently, sediment transport is limited by abundant woody debris, causing channels to aggrade and bury gravels. We evaluated the impacts of a wood debris and beaver dam removal remediation strategy on fine sediment transport and exposure of the underlying gravel and cobble substrates in a second-order Lake Superior tributary. A 300-m treatment reach received selective wood removals and was compared with both an upstream 300-m reference reach receiving no alteration and a downstream reach to monitor the effects of transferred fine sediment. Physical channel measurements were taken before the wood removal process, with repeat sampling at 10 and 12 months posttreatment. The wood removal treatment resulted in a significant 25% narrowing of mean stream widths, a 32% increase in mean flow velocities, a 58% reduction in the sand bed load, and a 400% increase in the available coarse substrate across the channel. Grain size distributions coarsened noticeably from pre- to posttreatment. Water depth and temperature were not altered by the treatment, and the reference station physical measures were relatively unchanged over the 12-month study period. Immediately after the wood removal there was an observable pulse of sediment traveling through the downstream reach that increased the sand bed load, increased stream widths, and filled pools. However, these impacts diminished with time, and near-preremoval conditions had returned after 12 months. The wood removal treatment caused a significant reduction in sand content and enhanced the availability of salmonid spawning substrates.