River Corridor Mapping Method for the North River Watershed, Colrain

Through a recent Innovation Grant from the MA Executive Office of Energy and Environmental Affairs, the Franklin Conservation District (FCD) has been able to undertake an important project to benefit the Deerfield River watershed. FCD has partnered with Field Geology Services to develop a protocol to identify river areas most susceptible to erosion and apply it to the North River watershed in the Town of Colrain. This river corridor mapping will provide a cost-effective tool for towns, state and federal agencies to identify vulnerable locations at a municipal, parcel-level scale. Protecting those areas along the river where erosion is most likely to occur will improve ecological function, limit downstream flooding and sediment loading, and increase community resiliency to future climate change.

The North River, a tributary of the Deerfield River, suffered some of the worst damage from Tropical Storm Irene in 2011, including severe erosion of agricultural fields, streambanks, landslides, road washouts and damage to bridges. Severe bank erosion continues to threaten Colrain community wells and public wells for Shelburne and Buckland that are located in Colrain.

To place this storm in context, UMass researchers have estimated that sediment discharged from the Deerfield River in one day exceeded, at least 10 to 40 years’ worth of normal sediment discharge and accounted for approximately 40 percent of the total sediment discharge from the Connecticut River resulting from Irene.1 They also reported that Irene caused the most severe erosion of any flood in the historic record. They attribute this extreme erosion to the fact that at the time of the storm soils were saturated due to higher than normal rainfall. The researchers conclude that the potential for highly erosive storms is growing as wetter conditions increase due to climate change.2

1 Yellen, et al., 2014.Source, conveyance and fate of suspended sediments following Hurricane Irene. New England, USA. Geomorphology v. 226, p. 124-134.2 Yellen, et al., 2016. Historically unprecedented erosion from Tropical Storm Irene due to high antecedent precipitation. Earth Surf. Process. Landforms, Wiley Online Library, DOI: 10.1002/esp.3896.

Towns affected by Irene have worked to repair damages, but staff and financial resources of our small, rural towns are very limited. Communities would benefit from help to identify areas that are most susceptible to erosion in future events. They would also be helped by strategies and tools to protect and restore these locations.

The goal of this project is to develop and pilot an innovative river corridor mapping tool that will identify areas susceptible to erosion and channel migration and help prioritize river and floodplain restoration projects and river corridor protection opportunities. A key objective of the project is to develop a tool that can be applied in communities across the Commonwealth and beyond. It will also inform the development of a new paradigm for river restoration; namely, land conservation of erosion and flood-prone areas.

A great need exists for an efficient method for identifying the area along a river that is most susceptible to erosion due to long-term channel migration or sudden shifts in channel position resulting from log jams, ice jams, or catastrophic flooding. Federal, state, and local officials need such information to identify at-risk infrastructure, but existing methods are either too coarse for detailed planning or too time consuming and expensive to be of practical use for most towns experiencing frequent erosion-related damages to infrastructure. Protecting those areas along the river where erosion is most likely to occur over long time periods – decades to a century -- has the added benefit of improving ecological function, limiting downstream flooding and sediment loading, and increasing community resiliency to future climate change.

Field Geology Services will develop and test a method for delineating river corridors that will be of practical use to municipalities and will significantly reduce costs compared to current methods. The focus area in the North River watershed will include reaches where erosion threatens public wells for Colrain, Shelburne and Buckland, areas of great concern for the communities and FCD.

The North River represents an excellent location to conduct a pilot study for this method because the significant erosion damages experienced during Tropical Storm Irene in 2011 led to several studies attempting to identify at-risk areas and potential restoration projects to reduce future damage. The data collected during these studies will provide a means for checking this

approach for delineating corridors before extending the work, as part of future projects, to the greater Deerfield watershed and beyond.

Field is delineating river corridors by using remote sensing data (e.g., aerial photographs, topographic maps, and

LiDAR) to determine the outer limits of river migration in the past as evidenced by abandoned channels, oxbows, crescent-shaped edges of low terraces, and other visible features indicating where the river has been in the past. Mapping tools are used to draw a line along these outer limits to define the river corridor on either side of the river channel. The corridor limits are being refined by viewing historical aerial photographs and topographic maps that may show previous locations of the channel before such evidence became less distinct with time. The corridor limits established from remote sensing data are being field checked for accuracy.

The identification of river corridors will also reveal those areas most likely to experience rapid changes during a flood and thus highlight high priority areas for conservation and protection from future development.

Land conservation must be viewed as a new paradigm in river restoration. The current focus on engineering projects leads to immediate but often unsustainable improvements in river function that are locked in place and unable to adjust to future climate change. Corridor mapping provides a framework for identifying a new class of restoration project that allows rivers to more freely migrate through portions of a corridor thus increasing resiliency to climate change and protecting downstream areas with at-risk infrastructure.

Franklin Conservation District’s river corridor mapping and protocol are being further advanced by the Franklin Regional Council of Governments’ work to create tools to accompany the mapping in the form of a model River Corridor Protection Overlay Zoning District Bylaw and a River Corridor Easement being developed in partnership with the Franklin Land Trust.

These three pilot projects – river corridor mapping, a model river corridor protection overlay district and model river corridor easement tool form a complete toolkit to create an effective and efficient model for natural resources conservation that can be applied throughout the Commonwealth. FCD will work beyond the grant period to engage communities in working with the mapping protocols and the associated land protection tools for river corridor management.

Photo: Field Geology Services

Photo: Field Geology Services

Image: Vermont Flood Ready

Image: Field Geology Services