This lake was previously a large, year round lake. According to historical records, Benton Lake, which is less than 3 miles northwest of Black Horse Lake, was used by early European settlers in the Great Falls, Montana region, to irrigate farms. A canal or canals were dug to drain Benton Lake for irrigation. The one remaining canal leads from northwest to southeast, directly to Black Horse Lake. The canal abruptly stops about a third of a mile from where Black Horse Lake was situated. Benton Lake is 3,625 feet (1,105 m) above sea level.
The present size of Black Horse Lake varies due to varying precipitation and other factors. In dry years, the lake never fills up at all. The lakes maximum extent is 1.5 miles from north to south, and 1.25 miles from east to west covering an area of 3 square miles (7.8 km2)