Otisco Lake is approximately six miles (9.8 km) long. The outflow enters first Onondaga Lake via Nine Mile Creek and then Lake Ontario via the Seneca River and Oswego River. The lake is the source of water for a district west of Syracuse.
At its deepest point the lake is approximately 60 feet in depth. Part of Otisco Lake is man-made, due to raising the water level by damming the outlet into Nine Mile Creek. The dam initially was constructed to provide a water reservoir for the Erie Canal. The water level was raised nine feet at the time. Subsequently the dam was rebuilt, raising the level an additional four feet. The natural part of the lake, closer to the southern end, is the deepest. A causeway across the lake was built to carry a road, flooded when the dam raised the level. The road no longer is used. A break in the causeway allows boats to pass between upper and lower parts of the lake. Although not maintained as a public recreational facility, the causeway is much used as a fishing spot.
Closest of the Finger Lakes to the City of Syracuse, the lake is extremely popular in the summer and experiences a population explosion every Memorial Day. On the first day of bass season, usually around the end of the second week in June, the lake fills with boats for the annual fishing derby. Campsites and houses are available for rent, and there is a county park for public use. The Otisco Lake link below mentions three public boat launching sites. There is also a commercial marina on the east side.
According to the U.S. Environmental Protection Agency, "the lake is affected by nonpoint source pollution from agriculture (pesticide and fertilizer use, manure production, and sedimentation resulting from poor tillage practices), residential land use (septic systems, lawncare, and construction), and streambank erosion." Currently the lake is experiencing severe problems brought on by zebra mussels which have come into the water via boaters. Swimmers should wear appropriate footwear as the mussels can be quite sharp.