Hemlock Lake is one of a group of long and narrow lakes located in western New York State known as the Finger Lakes. The basins of these north-south oriented lakes were formed by the advance of ice masses during the Ice Ages, and further sculpted by glacial meltwater during the interglacial and postglacial periods.

Although Hemlock Lake is relatively small, it possesses the proportions characteristic of the Finger Lakes with a length of 10.8 km, mean width of 0.7 km, mean depth of 13.6 m, and a very steep slope of 7.8%. Although the Finger Lakes region is now well-known for its many vineyards and orchards, it had been densely forested up until the early 1800's when large areas were cleared for agriculture by white settlers. The forests are now in the process of expanding as marginal farmland is abandoned and gradually invaded by trees.

Of Hemlock Lake's 96 km2 drainage basin, approximately two-thirds is still forested, one-fourth is actively farmed, and 1% is residential. The climate of Hemlock Lake is characterized by cold, snowy winters and warm, dry summers; and the lake regularly freezes over in the winter. The lake has served as a water supply for the city of Rochester for more than 100 years. The city owns all of the lakeshore property, and maintains strict control over the watershed area. In order to protect this water supply, cottages were removed for some distance back of the shores, the water level was raised and stabilized, and boats and motor sizes were limited (1).