USING GEOMORPHOLOGY TO GUIDE STREAM RESTORATION ON NASH STREAM, NEW HAMPSHIRE

Stream restoration on Nash Stream in northern New Hampshire is being undertaken to address habitat degradation associated with decades of log drives and a large dam break flood in 1969. Large boulder bars deposited by the flood are constraining the channel both vertically and laterally and inhibiting regrowth of riparian vegetation. Post-flood and earlier channelization and straightening removed boulders, logs, meanders, and other elements of structural complexity along the channel. The channel morphology resulting from the decades of human-induced channel alterations is a wide shallow channel with few pools, limited cover, armored channel bottoms, and limited capacity for self-adjustment, creating poor habitat conditions for Brook trout and other aquatic organisms. Improvements to morphological and habitat conditions would improve only slowly without restoration.

The numerous restoration techniques employed on Nash Stream are returning structural complexity to the channel while removing constraints to channel adjustment. Excavators are being used to: 1) remove berms from the top of the banks and reconnect the channel to the adjacent floodplain; 2) construct a variety of boulder clusters and log jams to encourage pool development and meander reformation; and 3) reconnect abandoned meandering side channels where in-stream wood, riparian canopy, and other important habitat features are already present. In areas difficult to access with heavy machinery, whole trees are being loaded in the channel at upstream locations and allowed to migrate freely through the channel where they have clustered together as log jams that form meanders, scour pools, deposit spawning gravels, and create cover. By restoring natural channel processes rather than imposing a final morphological condition on the channel, improvements in pool depth, flow complexity, and other habitat conditions on Nash Stream will be sustainable and the stream will have a greater capacity to adjust to changing watershed conditions resulting from climate change or other perturbations.