OCR Text |
Show Dorothy Sack, Department of Geography, Ohio University, Athens, Ohio, 45701 INTERLAKE FLUVIAL LINKAGES IN GREAT BASIN AQUATIC HISTORY: EXAMPLES FROM LAKE BONNEVILLE SUBBASIN INTEGRATION For more than a century, scientists have periodically published comprehensive inventories of the late Pleistocene lakes of western North America ( e. g., Russell, 1885, 1896; Meinzer, 1922; Hubbs and Miller, 1948; Snyder et al., 1964; Mifflin and Wheat, 1979; Williams and Bedinger, 1984). Although these inventories have been conducted within various specific physiographic or political boundaries, each includes all or a large part of the Great Basin. Information provided about Great Basin paleolakes in that literature ranges from approximate location and relative extent, as delineated on small- scale maps, to measurements of such lake attributes as depth and area, to observations regarding possible overflow episodes for the predominantly closed- basin lakes. Not surprisingly, those contributions emphasize the maximum extent of the paleolakes and any associated overflow which made the lakes tributary to lower closed- basin lakes ( or the sea) via connecting stream courses. That emphasis, however, obscures the fact that most large paleolakes, such as Lakes Bonneville and Lahontan, were assemblages of several conjoined subbasins united at higher water levels into a single lake. Many lacustrine subbasins, moreover, which constituted arms or embayments of the major lake at its higher levels, contained independent closed- basin lakes before and after their interval of integration. In order to reconstruct accurate paleohydrologic and paleoclimatic models for a region that contained closed- basin lakes, it is essential to distinguish those water- level fluctuations controlled largely by geomorphic factors from those induced by climatic variables. A shoreline that is well represented in the geomorphic or stratigraphic record, for example, is not necessarily the result of static climatic conditions; nor does a change in lake level always indicate a climatically induced shift in effective moisture. In addition to climatic factors that affect lake hydrology, geomorphic factors influencing lake hydrography are also reflected in the lake's shoreline record. Variations in lake size that create multiple shorelines may be the result of changes in drainage basin area due to such factors as tectonism, volcanism, mass movement, stream diversion, or overflow of upstream lake basins. If the geomorphically induced water- level change is rapid, a vertical zone largely devoid of shoreline evidence may result. It is also possible that a well- developed shoreline in a closed- basin lake marks the elevation of an intrabasin threshold rather than an interval of climatic equilibrium ( Eardley et al., 1957; Benson, 1978; Benson and Paillet, 1989). For example, when, under conditions of increasing effective moisture, the water level in one subbasin reaches the elevation of an intrabasin sill, it may remain essentially at that elevation until overflow fills the adjoining closed basin to the threshold level and a single integrated closed- basin lake is formed. The coalescing of subbasin lakes into a single water body by successive overflows from one subbasin to another can create a complex set of shoreline evidence. Properly allocating the contributions to paleolake- level changes made by geomorphically controlled versus climatic variables leads to a more accurate reconstruction of the basin's paleoclimatic and geomorphic history. The Bonneville lake basin consists of seven major closed subbasins each of which was integrated into the megalake for a different period of time. Lake Bonneville originated in the lowest subbasin, that of Great Salt Lake. Rush Valley was part of the Great Salt Lake basin at the start of the Bonneville lacustral cycle but was later separated from it by the Stockton Bar complex, which was built across their connecting strait ( Gilbert, 1890). Integration of the Great Salt Lake Desert, Puddle Valley, and Tule Valley occurred when transgressing |