In Illinois, the shells of fresh-water mussels are often found in large numbers in the middens of former Indian habitations which were located adjacent to rivers. Living representatives of the species present are usually common in the rivers of the state today. With knowledge gained through study of the habitat-demands of the different species as they now live, one is able to reconstruct the environment in which each species existed at the time when the mussels were taken for food. The picture formed by the combined habitat-demands of the different species is a portrayal of the river as it existed at that time. An analysis of the shells taken from a midden on Haw Creek, now often intermittent during summer months, suggests that it was a small river 2000 years ago. Other analyses of shells have shown that decided changes have taken place in other streams.