Seventeen seasonal ponds in Pennsylvania containing the federally endangered northeastern bulrush, Scirpus ancistrochaetus, were sampled for water chemistry, hydrology, and plant population area. [Na], [Mg], [Ca], [NH₃-N] and pH increased with decreased water level. [Mg], [Ca], [NH₃-N], [PO₄] and [K] were correlated with [Na], suggesting that they were controlled primarily by physical factors. Both surface and subsurface water contribute to water supply with five ponds dominated by surface water and four others dominated by subsurface water. Eight ponds show fairly equal inputs of surface and subsurface water during spring and fall, with subsurface water input becoming more important in midsummer. The difference between inputs of surface versus subsurface water was significantly negatively related to the percent change in population area during dry years and showed a positive trend in these two variables during wet years, indicating the sensitivity of this species to changes in surface water inputs.