As the glaciers began to decayor during any of the brief warming eventsthe rivers began to flow again, though the outflow from Lake Great Falls remained blocked.  The thickest ice persisted for hundreds of hears.  The lower shorelines (at about 3500 ft elevation) of the lake near the city of Great Falls are very pronounced, and indicate that the lake may have remained at this level for thousands of years.  Unlike Lake Agassiz and some other large periglacial lakes, Lake Great Falls was not simply an artifact of the melting of the glaciers.  Its ultimate source was in the mountains, and it may have persisted throughout much of the Ice Ages.
After the catastrophic drainage of Lake Great Falls its shoreline receded rapidly, particular in the Plains where the lake was never very deep.  The lake was still large, but it is shown here as being cut in two parts.  This is because there is a ridge of elevated land north of the Sun River at about the same elevation as the spillway.  This ensures that any time a glacier dams up the flow of the Missouri and reforms Lake Great Falls, the upper side of the "funnel" is cut off at the mouth.
Whether a separate northern segment of the lake persisted is not known with certainty.  A broad shallow channel (U) probably remained to connect the two lakes. Unfortunately most of the evidence that might clarify the picture of low lake levels has been erased by subsequent glaciations.
As long as an ice dam existed and the lake level remained above 3100 feet,
it would have continue to overflow through the Shonkin Sag, continuing
to erode that valley.  The relatively shallow gradient of the lower
Shonkin Sag may be evidence that a large river flowed here for very long
times, but the flow was not extremely strong.
Next, the ice retreats