Climate factors such as precipitation and temperature as well as climate-driven geomorphic processes such as glaciation affect rates of erosion and the amount of sediment supplied from a drainage basin (sediment yield). Thus, sediment yield rates provide paleoclimatic implications, although very little work has been published on ancient sediment yield rates. Late Pleistocene Lake Bonneville (Utah, Idaho and Nevada), with its well-documented lake level history and well-preserved deltaic sequences, provides a unique opportunity to estimate ancient sediment yield rates. Two end-member delta systems, coarse-grained (gravel) classic Gilbert-type deltas and fine-grained (sand, silt and clay) deltas, are exposed along the eastern margin of paleolake Bonneville. One example of each end-member delta is examined in this study (American Fork and Weber River deltas). The established Bonneville hydrograph can be used to determine time of deposition for a given package of sediments because these shoreline deltaic sediments were deposited in shallow water. The geomorphic expressions of these deltas are well-preserved and their sediment volumes can be determined. River drainage basin area can be combined with time constraints and delta volumes to estimate sediment yield rates. Sediment yield rate estimates for the coarse-grained American Fork and fine-grained Weber River deltas are 478 m(3) km(-2) yr(-1) +/-34% and 705 m(3) km(-2) yr(-1) +/-15%, respectively. The similarity of these values is somewhat surprising due to the large difference in drainage basin sizes (American Fork approximate to 160 km(2) and Weber River approximate to 3328 km(2)). However, this similarity suggests that precipitation may be the dominant control on sediment yield. Using sediment yield/precipitation regression equations developed for similar modern basins in the Southern Alps of New Zealand, a first-order approximation of paleoprecipitation is 1.6 m/yr (+/-14% for American Fork delta) and 1.9 m/yr (+/-7% for Weber River delta). These paleoprecipitation value are comparable to modern precipitation values of 0.4-1.5 m/yr (range of mean values over both drainage basins). A maximum late Pleistocene precipitation increase of 33% over the modern day value is suggested. This is determined by the difference between the highest paleoprecipitation rate (1.9 m/yr+7% of the Weber River delta) and the modern value (1.5 m/yr). Previous climate models and this 33% paleoprecipitation increase, suggest a maximum decrease in basin floor paleotemperature of approximately 13 degrees C. This paleotemperature decrease is consistent with previous estimates of late Pleistocene temperatures (using amino acid epimerization rates and periglacial wedges) in the Bonneville Basin and adjacent areas. These estimates suggest that although late Pleistocene precipitation rates may have been slightly higher than present (up to 33% higher), late Pleistocene temperature and evaporation rates were more important in the development of Lake Bonneville and associated glaciers.
