Standing‐wave measurements are discussed which show that the bottom of a fresh‐water lake near Washington, D. C. behaves acoustically as a pressure‐release surface between 50 and 500 cps. A correlation technique is presented which makes it possible to distinguish between the part of an acoustic signal which arrives by a direct path and the part which experiences a bottom reflection. Results of a study of the acoustic behavior of the lake bottom by this method confirm the conclusions reached from the standing‐wave measurements for frequencies up to 1600 cps. Correlation and transient measurements of the acoustic velocity in the bottom sediment are presented which indicate that the pressure‐release characteristics result from an acoustic velocity in the bottom of less than one‐tenth that of water. This low velocity is attributed to the presence of gas resulting from decaying organic matter in the bottom. The gas greatly increases the bulk compressibility of the bottom sediment and hence lowers the sound velocity.