The St. Joseph River runs nearly diagonally through the township of Constantine, from N. E. to S. W. A small affluent, called Fawn River, empties into it at the point where the town itself is located.

The characteristic vegetation along the banks of the main river above town is beech, maple, elm, iron-wood, walnut, butternut, papaw, magnolia (or a tree very nearly allied), spice-bush, prickly-ash, sumach, witch-hazel, wild grape, ash, oak and hickory, with many quite rare and beautiful wild flowers, such as are found in rich and favorable places. But that of the affluent region is almost entirely oak and hickory (mostly oak), with hazel, sumach, a few poplars, willows, and in some places cedars, but with few wild flowers of any account. The cedars are also found upon the main river, both above and below town. This difference in vegetation is probably due to a difference in the nature of the soil, the lower and richer soil of parts of the main river producing a richer and more varied flora than the higher and poorer parts adjoining the affluent.