Glacial-lake spillways in the midcontinent region consist of narrow, deep trenches connecting proglacial lake basins. Geomorphic and sedimentologic evidence from the Souris and Des Lacs spillways in Saskatchewan and North Dakota indicates a catastrophic drainage of Glacial Lake Regina. The Souris spillway in Saskatchewan is composed of a broad, upper scoured surface 5 to 10 km wide, which merges with the floor of Glacial Lake Regina. The upper surface is longitudinally grooved and mantled with coarse lag deposits. An inner trench, 1 km wide and 20 to 45 m deep, occupies the center of the spillway. The two-level morphology of the spillway probably developed by initial erosion of the upper level, followed by incision of the inner trench, a sequence of events shown by flume experiments performed by Shepherd and Schumm. Near the Saskatchewan and North Dakota border, the flood water bifurcated into two branches as part of the water spilled over a divide into another spillway channel. The two spillways in North Dakota, pre-existing meltwater channels, were occupied and enlarged by the discharge from Glacial Lake Regina. Along the path of the spillways leading to the Glacial Lake Souris basin, water repeatedly breached the sides of the channels, eroding shallow channels leading away from the spillway. Near Minot, North Dakota, a significant amount of the water spilled out toward the east and eroded a plexus of anastomosing channels with streamlined erosional remnants between the channels. Deposits of the flood include huge point bars located at the inside of each channel meander. Internally, the bars generally consist of poorly sorted, indistinctly bedded, coarse gravel containing boulders of resistant and nonresistant lithologies as much as 3 m in diameter. Peak discharge of 1 × 105 m3/s or more may have been achieved at the outlet of the lake. At this rate, Glacial Lake Regina would have drained rapidly, probably in less than a month. The most likely cause for the rapid drainage of such proglacial lakes as Glacial Lake Regina was high discharge inflow of water from an upstream source along the ice margin.