MONITORING SUBSURFACE WATER FLOW THROUGH AN EARTHEN DAM USING ELECTRICAL RESISTIVITY, RICE CREEK FIELD STATION, OSWEGO, NEW YORK

In 1963-1965, an earthen dam was constructed on Rice Creek, Oswego County, New York, to support field research related to the local field station (SUNY Oswego). The dam spans the width of the original creek channel, includes an overflow drain pipe, and a spillway. Although engineering records are incomplete, the core of the dam appears to consist of meter-scale blocks of stone and sediment fill. The dam is about 75 meters long and 25 meters wide (including material below the water). From vintage air photographs, it’s clear that the dam was constructed to flood an existing wetland and create the 14 acres Rice Pond. A recent survey of the pond estimates the average depth to be about 2 meters, therefore the dam holds about 8000 cubic meters of water. In 2010, a program to assess the integrity of dams on Rice Creek began and resulted in the removal of a dilapidated cement and stone dam upstream of Rice Pond. At that time, Wenner ERT surveys were conducted across the length of the dam using an automated Junior Syscal switching ER meter, 24 electrodes, 3 meters spacing, to image the entire dam and the native materials below the dam. Early surveys were completed when the creek discharge was low in late summer, and no water in the overflow drainage that passes through the dam. The burial depth of the drain pipe (construction report) provided sufficient information to convert ER pseudosections to true depth. As surveys progressed weekly through the fall months, a substantial ER low anomaly developed at about 4 m depth near the SW end of the dam, and the anomaly correlated with observed seepage on the downstream side of the dam. Further ER images showed expansion fo the low ER anomaly as the level of the pond increased through the rain and snow ridden months of late fall and early winter. This experiment was repeated again in the late summer and fall of 2018, and showed the exact same results. The low ER anomaly progressively expanded toward the southwest (left bank of filled creek channel beneath dam) and remained relatively fixed in position toward the middle of the dam. Additionally, the expansion of the low ER anomaly correlated with expansion of the seepage areas at the base of the downstream side of the dam. Clearly, as the pond water level increases, thereby increasing the internal water pressure on the dam, the original creek channel, now filled with large rocks and sediment, is a continuous conduit for water flow through and under the dam. Monitoring the dam on Rice Creek will continue for several more seasons with the objective to determine the volume of seepage.