Two new methods for the estimation of the reaeration coefficient in a stream are derived. The first method, termed the “peak lag method,”; or PLM, calculates the reaeration coefficient, K, using the observed time lag between solar noon and the maximum afternoon dissolved oxygen concentration. Computer simulation demonstrates that this technique is less accurate for small values of K or short day length. By using a correction term calculated from the observed time lag and the day length, a second technique, termed the “modified PLM,”; is developed which has improved accuracy in these situations . These methods require only observation of the time of the peak dissolved oxygen concentration and may therefore be more easily applied in some situations.