Erosion is a natural process that, in free-flowing rivers, reaches a state of equilibrium: the amount of sediment lost from one area equals the amount of soil deposited in another area. This natural cycle of erosion and sedimentation is beneficial to the aquatic ecosystem. Gravel beds are created for fish spawning and a series of pools, riffles and runs also provide habitat. Human use of the river and land within its watershed that increases the rate of erosion and sedimentation can result in the loss of aquatic habitat, valuable agricultural soils, damage to roads, buildings and other structures, and pollutant-laden sediment.

The causes of bank erosion are complex and interrelated and include: river flow velocities, water level fluctuations, bed and bank material, pressure imbalances at the bank face, absence of bank vegetation, obstacles in the stream, waves and boat wakes, freeze-thaw and wet-dry cycles, and ice and debris.

Banks comprised of sands and silt, like those in the Turners Falls Power Pool, are easily eroded while cobbles, which are heavier, and clay, which is more cohesive, are more difficult to dislodge. Pressure imbalances along the bank face can be the result of groundwater seepage or the rapid drawdown of water. For example, when the water level of a river is high for a period of time, water tends to seep into the bank. When the water level drops rapidly, the bank materials can not drain as fast, and the remaining water in the bank face increases the outward pressure on the soil and reduces its stability. Riparian vegetation, such as trees, shrubs, grasses and herbaceous plants, reduces the velocity of surface water runoff, helps to filter pollutants, and provides resistance to erosion. Unvegetated banks and those comprised of sands and silts are also susceptible to undercutting by water level fluctuations, waves, and boat wakes.

Bank Stabilization Projects in the Turners Falls Power Pool
The banks of non-cohesive, alluvial sand and silt which dominate the Turners Falls Power Pool section of the Connecticut River typically exceed twenty (20) feet in height. Erosive forces have destabilized many sections of bank resulting in slumping and mass wasting of large sections of bank and the loss of trees and other riparian vegetation on the top of the banks. Over the years, various efforts have been undertaken to reduce the loss of agricultural land, riparian buffer zones, archaeological resources and private property. In the mid-1970's, following construction of the Northfield Mountain Pumped Storage Facility, Northeast Utilities made efforts to stabilize major segments of the river bank using tree clearing and hydroseeding and some rip-rap revetment. The Army Corps of Engineers (ACOE) used several techniques, which were considered innovative at the time, including: precast cellular concrete block mattress, used auto tire wall and used auto tire mattress. All three treatments included vegetative protection on the upper bank.

The license to operate the Northfield Mountain Pumped Storage Facility is issued by the Federal Energy Regulatory Commission (FERC). The language in the license, specifically Article 20, makes the operator of the facility responsible for any soil erosion and siltation that occurs in the Power Pool as a result of construction and operation of the facility. In an effort to comply with their license requirements, NU hired Northrop, Devine & Tarbell to prepare a Master Plan that would inventory and assess riverbank conditions in the Power Pool and recommend a plan of action for mitigating erosion. The draft of this report was issued in June 1991 and was deemed unsatisfactory by many of the stakeholders. There were strong objections to the proposal to stabilize eroding bank with rip-rap and other "hard" engineering techniques because of concerns about the aesthetics, the lack of native vegetation, and the loss of wildlife habitat if these techniques were used. The proposed work was ultimately abandoned when state regulators refused to issue necessary permits for the work.