1 84 Branley A. Branson 



The Interior Low Plateau is a unique province in many ways. The 

 region has been periodically convulsed by tectonic readjustments of the 

 crust that stimulated some reorganization of drainages. It was also 

 influenced by the formation and maintenance of the Mississippi Embay- 

 ment and the isthmus of stable land at the northwest corner of the Low 

 Plateau, and /or by eustatic adjustments of the Embayment, and by 

 stream evolution and modification in extralimital areas, mostly taking 

 the form of headwater piracies between various streams. Pleistocene 

 glaciation caused enormous changes by obliterating much of the old 

 Teays River system and other streams in the unglaciated area, and by 

 successively turning streams westward to form the Upper Ohio River, 

 connecting streams that had never before been in direct contact. This, 

 and the placement of the Mississippi Embayment, conspired to form 

 one of the most unusual hydrologic phenomena in North America. More 

 large rivers find confluence near the junction of Illinois, Indiana, Mis- 

 souri, and Kentucky than at any other area of equal size in North Amer- 

 ica: the Ohio, Mississippi, Tennessee, Cumberland, Green, and Wabash 

 rivers. Some of these streams run in nearly ancestral basins, but others 

 do not. The biological function of all these changes was, of course, to 

 open up faunal exchange pathways that had not previously been avail- 

 able. The influence of the Mississippi-Ohio connection upon the repopu- 

 lation of Indiana and Illinois segments of the Wabash drainage has been 

 very great. During meltback, many small streams were doubtless exter- 

 minated and larger ones modified by outwash and deposition, judging 

 from the thickness of known deposits in Indiana and adjacent Kentucky 

 and the fossils contained therein. 



Stream-margin and tributary migration ("hopping"), both upstream 

 and downstream, is still a viable hypothesis to explain some observed 

 distributional patterns in small-stream (third order and smaller) species. 

 The meltback of even enormous glaciers is not a constant phenomenon, 

 but one that varies according to season and even time of day, creating 

 impulses of high- and slack-water conditions. Where warmer water 

 flows into glacial streams, the warm water does not mix immediately 

 with the cold. Instead, as I have observed at the Athabascan Ice Field in 

 Canada and at many smaller valley glaciers in British Columbia, 

 Washington, and Oregon, the warmer water flows along the margins of 

 the current. Thus, in the case of Interior Low Plateau drainages, various 

 species of fishes could have found their way into tributaries where they 

 had not previously occurred. 



There were, of course, fishes already present in Low Plateau 

 streams prior to the onset of these influences; some of those remain as 

 relicts, either as endemics or as segments of the fauna that are identical 

 with or closely related to segments elsewhere with varying hiatuses 

 intervening. Newly arrived forms from elsewhere may have, because of 



