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93 Every time the glaciers ground their way south, any critters living in the path of the ice would have been wiped out. Then, as the glaciers melted and retreated back north, new streams and watersheds formed and crayfishes could follow along and colonize these new waters. But, to be able to colonize the new streams, wildlife including crayfish, had to survive in places of refuge (called refugia) away from the ice and the harsh conditions near the ice. Several authors have already looked at where these refugia may have been located. 18, 110, 180, 213, 233 These authors suggest that the refugia of the Northern crayfish, the Calico crayfish and the Prairie crayfish were in the Missouri River basin to the west. The refuge of the Devil crayfish must have been to the south of the ice margin. The point is that if we plotted the ranges of these species on a national map and then marked where the margin of the ice had been, we might be able to trace these refugia. We could also plot the current Nebraska crayfish distributions on top of the state glacial till/loess map. This may help us trace how they could have colonized the new drainages and, perhaps, there are also clues as to the landforms that produce the habitats they prefer. It might be helpful to review the timeline of the formation of some of the major rivers of the state shown in the map above. The Platte and Republican Rivers are the oldest streams in the state and existed before the Pleistocene began. The Republican River flowed southeast in the same general area where it is now. The Platte was flowing northeast then migrated into a southeasterly course as part of the ancestral Grand watershed flowing through central Missouri. After the Platte had moved to the southeast, the Loup, Elkhorn and Blue River basins must have begun forming. In the late Pleistocene, the Platte was diverted into its present course. At the end of the Nebraska river basins