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51 Ozark Highlands. 30 It was thought that the isolated populations in north-central and northwestern Kansas, northeastern Colorado, and southwestern Nebraska represented relict populations. It was felt that these were the remnants of a much larger range that extended through a large drainage system that flowed east and south through central Kansas during the Pleistocene glaciations. Following glacial retreat, the two population centers were disconnected. The hypothesis was that European settlement in the 1800's brought the Great Plains under cultivation which increased the siltation of its streams, making them unsuitable for the Ringed crayfish. As a result, most of the Great Plains populations were presumably lost. 72, 246 The plowing of the prairies had a negative impact on many species, including the Ringed crayfish, but my Big Blue River collections (where turbidities often exceed 500 ntu) is evidence that this species can tolerate turbid, silty waters. It is possible that this tolerance may represent an adaptation as the Big Blue River has not always been as turbid as we now know it. John Charles Fremont camped on the Big Blue on 20 May 1842 and on page 177 of his report he wrote that "This is a clear and handsome stream, about one hundred and twenty feet wide, running, with a rapid current, through a well-timbered valley". 71 That the Big Blue River was, historically, a clear stream is also noted on page 52 in a history of Gage County published in 1918. Here it was noted that ". . .before the wash from cultivated lands had changed their character its waters were clear, sparkling, beautiful as a mountain stream---in deep places as blue as the overhanging sky". 49 There are a number of fishes in central North America with disjunct distributions . 38, 39, 159 One of these, the Plains topminnow (Fundulus sciadicus), has a distribution that is strikingly similar to the current distribution of the Ringed crayfish. 38, 136 The existence of an "Ancestral Plains Stream" that formed when the Pleistocene glaciations diverted eastward-flowing Great Plains rivers to flow southward has been postulated. 159 Support for this is found in the distribution of the Plains topminnow. "..the modern distribution of Fundulus sciadicus suggests southeastward displacement of that species from a place of origin in the central plains into the northern and western parts of the Interior Highlands, where relict populations persist. The Ozarkian populations might have been established as early as the Kansan glaciation via the newly integrated Missouri River Basin or the Ancestral Plains Stream". 39 Further support for this hypothesis is found in a genetic study of the Plains topminnow which found that two widely separated populations (in Nebraska and in the Lamine River of Missouri) were once connected. 138 Given this information, it would seem that, if it was possible for the Plains topminnow to disperse southeast through this Ancestral Plains Stream, then it would seem to be equally possible that the Ringed crayfish could disperse northwest through the same system. Fish distributional data have been used to describe the hydrographic history of drainage basins. 160, 241 In the same way, the distribution of the Ringed crayfish may show us the nature of the Pleistocene and post-Pleistocene drainages in Nebraska The Ringed crayfish has been collected from the Republican, Big and Little Blue, Platte, Loup, and Niobrara River basins in Nebraska. During Illinoisan times (~200,000 years ago) the Republican River flowed east and southeasterly approximately where it is today. The North and South Platte Rivers also had merged into a