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'. . -" '.~..'~~~~':"" ~.~'::~::::~~ ..'::~=:..~~_:~~~._~~:~:=~:~~~~~~~~:~':.~:'::~:':".~ ~~...~ ...~.~:~..~ . :~'.~:~~l'~ <. I.. !! :''"'. ...... 0' .tala originally described, the original author's name is placed in paren- theses to signify that a change in generic placement has been made. All such alterations in scientific names and the attendant de- tails of their usage in nomenclature are made according to estab- lished rules and procedures published by the International Commis- sion o.f Zoological Nomenclature. The more serious student can be- come familiar with these procedures in nomenclature and in classi- fication by reading "Procedure in Taxonomy", by E. T. Schenk and J. H. McMasters, 3rd edition, 1956, Stanford University Press. major drainages There are but two important drainage systems in Utah, the Bonneville Basin and the Colorado River Basin. A long chain of mountains and plateaus, beginning in the northeastern part of the State, continues generally southward and then veers westward to divide Utah into nearly equal halves (Fig. 1). The western half lies in the generally arid Great Basin. Here the rainfall does not reach the sea but is collected in lakes or sinks on the valley floors, from which it is lost by evaporation. Less than 15,000 years ago, however, a vast body of water, called Lake Bon- neville, covered most of this part of Utah and for a time had an outlet to the north into the Snake River. Its major remnants are Great Salt Lake, Utah Lake, Sevier Lake, and Little Salt Lake. At its maximum development it had a depth of about 1,000 feet and an area of nearly 20,000 square miles. about ten times that of Great Salt Lake. Retreat of the glaciers and the attendant shift to a more arid climate resulted in the extinction of Lake Bonneville. The largest streams within this drainage are the Bear River in the north, the . Weber and Provo rivers in the east, and the Sevier to the south. The eastern half of Utah lies within the Colorado River Basin and comprises large, deeply eroded streams generally flowing from north to south. The two largest rivers are the Green and the Colo- rado, whose sources lie high on the snow-clad peaks of the Rocky Mountains. The steep gradient of the entire drainage, the silt load carried, and the great fluctuations in volume are noteworthy fea- tures and have enabled the rivers to cut through such imposing mountains as the Uintas at Flaming Gorge. The third largest river is the San Juan, which enters the State at the southeast comer and joins the Colorado just north of Rainbow Bridge National Monu- 26 - ~ ....:; -~... "