[en] The Los Alamos Scientific Laboratory conducted a Hydrogeochemical and Stream Sediment Reconnaissance for uranium in the Lamar National Topographic Map Series Quadrangle, Colorado. Totals of 1043 water and 734 sediment samples were collected from 1589 locations at an actual density of one sample location per 12 km² over a 19 400-km² area. Water samples were collected from wells, streams, ponds, and springs; sediment samples were collected from streams, ponds, and springs. Water samples were analyzed for uranium and 12 additional elements. The mean uranium content of all water samples from the Lamar Quadrangle is 17.63 ppB. The mean uranium content of well water samples is 19.42 ppB; this value is among the highest yet reported for the HSSR. Water samples from 21 wells contain more than 100 ppB uranium. Clusters of water samples containing more than 50 ppB uranium, the arbitrary anomaly threshold chosen for this report, are associated with the Pierre shale, Niobrara formation, Carlile shale, Greenhorn limestone, Dakota sandstone, and alluvium along the Arkansas River. Sediment samples were analyzed for uranium, thorium, and 41 other elements. The mean uranium content of all sediment samples is 7.2 ppM. Major clusters of sediment samples containing more than 21 ppM uranium, the arbitrary anomaly threshold, are limited to areas underlain by the Pierre shale. The highest uranium values and uranium/thorium ratios are associated with exposures of the upper member of the Pierre shale in the northwestern part of the quadrangle. In situ leaching of uranium might be feasible in areas in the southern half of the quadrangle where well waters have a high uranium content. The generally high uranium content for sediments associated with the upper member of the Pierre shale is an indication that intermediate-grade uranium resources might be present in this unit in the northwestern part of the quadrangle