Although the EPA said additional assessment activity was being conducted, company and tribal officials said the spill had been contained and did not affect the lake.
"We have a berm and a dike around it, around that bay area, to keep it from going into the lake," said Three Affiliated Tribes Chairman Tex Hall.
Saltwater is a naturally occurring, unwanted byproduct of oil and natural gas production that is between 10 and 30 times saltier than sea water. The state considers it an environmental hazard.
The byproduct, also called brine, may contain petroleum and residue from hydraulic fracturing operations.
Karolin Rockvoy, a McKenzie County emergency manager, said it was apparent from looking at vegetation that the spill went undetected for some time.
The number of saltwater spills in North Dakota has grown with the state's soaring oil production. North Dakota produced 25.5 million barrels of brine in 2012, the latest figures available.
Fort Berthold Indian Reservation plays a key role in the state's oil production, the second-highest in the nation. The reservation currently represents more than 300,000 of North Dakota's 1 million barrels of oil produced daily, according to the state's Department of Mineral Resources.
MacPherson reported from Bismarck, North Dakota.