ASSESSING THE IMPACT OF GROUNDWATER AND HETEROGENEOUS GLACIAL DEPOSITS ON STREAM BANK EROSION IN THE CATSKILL MOUNTAINS, NEW YORK

The Ashokan Reservoir supplies New York City with 40% of its daily fresh drinking water. The reservoir was formed by damming the Esopus Creek and is separated by a concrete dividing weir. Currently this water supply goes unfiltered, water enters the west basin and after a settling period is withdrawn from the East basin. The turbidity of the water entering the reservoir is increasing recently due to more dramatic flooding and significant precipitation events. This may lead to a need to filter the drinking water supply which could be very costly.

The turbidity problem can be attributed to the failing slopes along the banks of the Espous Creek, and other tributaries. While increasing frequency of severe storms is a factor it is not the single cause. Localized morphology and stream patterns has lead areas of high ground water pressure. These zones of ground water convergence create failures along the banks of the river. These banks consist of glacially derived clay material, which is the source of the suspended solids and geologic mapping was used to identify areas critical to the study. These areas are particularly prone to failure in high precipitation events because of the pre-existing ground water pressure.

To observe these patterns several failing slopes where surveyed using a clinometer, Jacob staff, and sight level. This data was entered into GIS to create a 1m by 1m 3D model of the slope. Geologic contacts, thought to be a contributing factor in the convergence of ground water were mapped into the model as well. Piezometers were placed at equal depths along a chosen elevation at equal intervals to monitor and collect ground water pressure data. At an active failure site, pressure head data remained consistently more significant towards the center of the failure over the course of the observation period than the pressure head data recorded at the fringes of the same failure. At an inactive site, over the course of the same period no pressure head data was observed. This data supports the theory of zones of groundwater convergence as a likely factor contributing to the failure of the slopes in question.