Bear River, the largest river in the Great Basin, had some of its flow diverted into Bear Lake through a series of canals constructed between 1911 and 1918, turning Bear Lake into a reservoir. The prediversion lake had an unusually high Mg2+ : Ca2+ ratio (38 by weight), which resulted in precipitation of CaCO3 as aragonite. The amount and mineralogy of the carbonate did not change immediately after the diversion, but during the middle of the 20th century, high-Mg calcite began to precipitate. In contrast, at the time of diversion there were very distinct changes in the isotopic composition of the carbonate that clearly define the time of diversion within the sediments. Sediment-trap studies show that the CaCO3 polymorph that precipitates in the epilimnion of the lake today is high-Mg calcite. Samples in sediment traps placed 2 m above the bottom of the lake in 45 m of water (bottom traps) contain predominantly aragonite and quartz, with lesser amounts of high-Mg and low-Mg calcite and dolomite. Isotopic evidence from the sediments shows that the aragonite that accumulated in the bottom sediment traps and that constitutes the bulk of postdiversion sediments is reworked and redistributed from shallow sites to deep sites. ?? 2007, by the American Society of Limnology and Oceanography, Inc.