An indirect method of determining flood plain aquifer parameters from estimated autocorrelation of groundwater table data is presented. For this purpose a stream-aquifer system is approximated by a cascade of linear reservoirs. The response of groundwater piezometric head to an arbitrarily varying flood pulse in the river is given by the convolution equation. The autocorrelation function of groundwater piezometric head is expressed as a function of the parameters of the aquifer and the autocorrelation function of the water stage in the river. Observations performed in Gaupne in western Norway are used to determine the parameters of a flood plain aquifer.