A model for bottom and subbottom scattering is proposed which estimates the short time average of the received level as a function of the processing bandwidth and the channel characteristics. The model also estimates the degrees of freedom available in the received level as a function of time. The model describes how the backscatter from individual modes decorrelates as a function of time and processing bandwidth. For narrow bandwidths, modes interrogate the same bottom patches at large time, giving structure to the expected received level. Narrow-band signals also interrogate more roughness correlation lengths, yielding lower expected fluctuations about the mean. Results from the model are compared to experimental observations and conclusions are drawn regarding the validity of the proposed approach.