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Erosion and Sedimentation

The Threat

All stream banks erode to some degree; it is an ongoing process that is incremental in streams that are stable and in equilibrium. Accelerated bank erosion is a symptom of a compromised stream and causes further erosion, sedimentation, and changes in channel structure – changes that can threaten human infrastructure and communities. The Ausable River is the second steepest river in New York State; it collects water from 512 square miles and descends almost 4000 feet from steep mountain tributaries to pass through fertile, low gradient valleys on its way to Lake Champlain. In moderate flows, severe bank erosion and channel deepening occur where the river encounters loose soils and raw banks compromised by the loss of vegetation or unsuitable development, such as undersized culverts or poorly placed hardening structures. Substantial flooding events exacerbate the most degraded sections of the river, toppling banks, compromising aquatic habitat, carving new channels, and redefining historic floodplains. The Ausable River is in disequilibrium. In order to address this cycle of degradation and implement natural stream restoration effectively, we need to know where to begin. Which sections of the river are most vulnerable, and how do we prioritize restoration work? 

AsRA's Response

In order to identify priorities for natural stream restoration, the Ausable River Association (AsRA) began measuring key indicators along sections, or "reaches," of the river in 1999. AsRA's staff walked the majority of the main channel of the river in 2006, assessing and cataloguing the extent and severity of erosion. Critical reaches are reassessed as needed. We use methodology developed by the Vermont Agency of Natural Resources (VANR) and described in their Vermont Stream Geomorphic Assessment Phase 1 Handbook. In this survey method, two measures are taken: the amount of bank erosion present in a given reach and bank height, measured from the streambed to the top of the bank or slope. Identifying the extent of bank erosion provides us with one measure of current vulnerability. Measuring bank height provides our second variable and is important for assessing the potential for bank failure and landslides. The VANR Handbook assigns ranks for the combined results of two measurements as shown in the "Bank Erosion Impact Rating" map, available in the PDF at left.

Identifying the problems and priority areas is the first step. In 2010, AsRA implemented a demonstration project at Intervale Lowlands Preserve on the West Branch of the Ausable that stabilized a badly eroding bank using woody material. The success of this structure was immediate - halting bank erosion, redirecting flows to the center of the channel, increasing water velocity - more efficiently moving sediment, and restoring habitat for fish and other wildlife. The project also solidified the strong partnership approach AsRA uses today as we continue to implement stream restoration projects throughout the watershed. AsRA's "stream team" includes US Fish and Wildlife stream restoration experts Carl Schwartz and  John Braico of Trout Unlimited. Our work also relies on communication with federal, state, and local permitting authorities, municipal partners, and landowners.

Since Tropical Storm Irene devastated the region in 2011, AsRA redoubled its efforts to restore priority reaches throughout the watershed. Recent restoration projects include: the Keene Town Beach weir replacement, Riverlands restoration, John's Brook restoration, Woodlea Farms stream repair, and Rivermede restoration.

What You Can Do

Ausable watershed landowners with property bordering any waterway can prevent erosion by maintaining a thick border of native plants and trees along streambanks. A general rule of thumb is to keep a border at least as wide as the waterway you seek to protect. The roots of plants stabilize soils and absorb a remarkable amount of water, reducing flood impacts. Avoid placing permanant structures, including walls or stacked boulders, on river or stream banks or armoring a bank with rip rap. While you may save soil directly under the rock, this simply transfers the problem: the areas upstream and downstream will become much more vulnerable to erosion.

If your streambanks are already eroding or collapsing, contact AsRA for advice.

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