Onondaga Lake has two natural tributaries that contribute approximately 70% of the total water flow to the lake. These tributaries are: Ninemile Creek and Onondaga Creek. The Metropolitan Syracuse Wastewater Treatment Plant (METRO) contributes 20% of the annual flow. No other lake in the United States receives as much of its inflow as treated wastewater. The other tributaries, which include Ley Creek, Seneca River, Harbor Brook, Sawmill Creek, Tributary 5A, and East Flume, contribute the remaining 10% of water flow into the lake. The tributaries flush the lake out about four times a year. Onondaga Lake is flushed much more rapidly than most other lakes. The lake flows to the northwest and discharges into Seneca River which combines with the Oneida River to form the Oswego River, and ultimately ends up in Lake Ontario.