We collected six samples of exposed streambed sand (250-850 μm) from the lower section of the Colorado River, along a ~450km distance starting south of Hoover Dam and ending near Yuma (see exact locations in data table). All samples collected were prepared at the Cosmogenic Laboratory, Institute of Earth Sciences, Hebrew University of Jerusalem, following standard procedures (Bierman & Caffee, 2001 (doi:10.2475/ajs.301.4-5.326); Kohl & Nishiizumi, 1992 (doi:10.1016/0016-7037(92)90401-4)). Be and Al isotopic ratios were measured at the accelerator mass spectrometry (AMS) facility (ASTER) at CEREGE, Aix-en-Provence, France, and normalized to accepted AMS standards.