ASSESSING THE INTEGRITY OF DAMS ON RICE CREEK, OSWEGO, NEW YORK, USING ELECTRICAL RESISTIVITY METHODS

In the 19^th and early 20^th centuries, many dams were built for power, water supplies and recreation, and many have outlived their primary purpose. At this point, these dams isolate segments of streams and rivers, and prevent migration of fish. In an attempt to restore stream habitats, some dams are under consideration for removal. Rice Creek is a small tributary that flows ~15 km across Oswego Co., NY into Lake Ontario. A small (20 X 4 m) stone-cement dam (Fallbrook) on Rice Creek is being investigated for removal to promote the migration of local native fish species. In 1965, a earthen dam (80 X 18m) was constructed on Rice Creek to form a pond as part of the SUNY Oswego Rice Creek Field Station. Although it is not being considered for removal, an investigation was funded by the Rice Creek Associates to assess the integrity of the dam. During this investigation, noninvasive electrical resistivity (ER) surveys were collected with the objective of identifying zones of seepage. During late summer and fall, 2010, Wenner ER surveys were conducted across the length of the dam using an automated resistivity meter with 24 nodes spaced at 3 meters to achieve penetration to a depth of 10.5 meters. The first survey was collected at a time when the creek discharge was low, and no water was flowing in the drawdown culvert that crosses through the dam. A very high ER anomaly correlates with the location of the culvert. Integrating the burial depth obtained from the dam construction report enabled us to identify the true depth of ER anomalies. During the early fall, an 18 X 5 m low ER anomaly located at a depth of 4 m appeared. Serial ER surveys were conducted from the upstream to the downstream sides of the dam to reveal that the anomaly passes beneath the dam. We interpret this anomaly as a zone of water seepage within the base and under the dam. As more ER surveys were collected through the fall rainy season, the anomaly expanded horizontally in the left-bank direction but remained unchanged toward the right. When compared with a 1964 air photograph of Rice Creek, there is good correlation of the original creek channel and the location of the ER anomalies. This relationship suggests that water is not seeping through the dam, but is seeping beneath the dam following the fill materials that were placed in the original stream channel during dam construction.