Brad Withrow-Robinson, Forestry & Natural Resources Extension Agent, Benton, Linn and Polk Counties.
We have written about forest diversity, its importance in providing habitat for different species, or a species at different seasons or at different stages of its development. We have addressed the forest, trees and shrubs in particular, but the importance of habitat diversity applies to other parts of the forest environment too. Like streams.
We learned about the importance of having different stream habitats to support fish, insects and other aquatic life while on a recent Extension tour. A stream can have many types of habitat. The anatomy of a stream (the stream’s morphology) can be described in terms that are familiar to anglers: pools, riffles, glides, bars and tail outs. Each term describes a different combination of water depth and flow that together provide a type of habitat. You can see and often hear this: Some parts are quiet (pool and glide), some gurgle their presence to those nearby (a riffle), while falls and a plunge pools announce themselves at a distance. Aquatic biologists get excited about streams with a good mix of these habitats in a stream reach, just as wildlife biologists get excited by forest structure and snags. Hmm.
The anatomy of a stream, like a forest’s structure comes about through the interactions of the landscape, natural processes and time. A narrow, steep valley largely constrains the stream to a defined place. A wider, gentler valley gives the stream room to wander and a chance to determine its own fate working with the rock, sand, gravel and logs to create riffles, bars and pools.
A diversity of stream habitats is important, explained Guillermo Giannico, OSU Extension Fisheries Specialist. There are many types of fish, amphibians and other creatures that live in our forest streams, and many of these animals’ have complex life cycles. Different species, ages and sizes have very particular and different needs for different habitat types (water conditions including depth and speed as well as temperature or clarity) to allow them to hide, rest, hunt, or spawn successfully. The more diversity there is in a section of a stream, the wider range of habitats are available for those animals to turn to as they develop and as the seasons progress.
Many common forest, farm and development activities have affected our streams and rivers over the years, giving them a simpler anatomy and less habitat diversity. Roads and culverts have also often blocked passage between stream sections and habitats.
This is the reason for much of the restoration work undertaken by landowners, watershed councils and others: to restore stream habitat diversity and foster the processes that drives its creation.
A key ingredient for habitat diversity in streams, as in many upland forest situations, is large wood. An old log decaying on the forest floor becomes habitat that shelters and feeds many different organisms. It does that in a stream too, but it also shapes its surroundings. Logs engage the stream each winter, altering and redirecting the current to dig into a streambed or bank here, accumulate sand and gravel there. Logs and other large wood are critical to the creative process of the stream, whether naturally occurring or placed there as part of a restoration project. It is the logs that dig pools, build bars, and lay out the riffles where fish live, anglers lurk and poets dream.
Large wood accumulated by an in-stream log placement by the Marys River Watershed council. Note channel cut next to bank and gravel accumulated down stream.