Sands of the Sagavanirktok River drainage basin, on the North Slope of Alaska, are composed primarily of calcite, quartz, and sedimentary and metasedimentary lithic fragments. The relative proportions of these components vary systematically through the drainage basin. Sands sampled in the river headwaters, in the Brooks Range, average 58% calcite, 35% lithic fragments, and 7% quartz. With increased distance from river headwaters, calcite is progressively removed from the sediments. Rivers sampled in the Arctic Foothills physiographic province average 29% calcite, 53% lithic fragments, and 18% quartz. Further transport results in enrichment of quartz relative to lithic fragments. Sands from the Arctic Coastal Plain average 9% calcite, 38% lithic fragments, and 53% quartz; the northernmost sample collected, from near the head of the Sagavanirktok delta, is 70% quartz, 5% calcite, and 25% lithic fragments. We attribute these increases in compositional maturity to chemical and physical weathering of sediments during temporary storage in alluvial sequences such as alluvial fans, river braid plains, point bars, and alluvial terraces. As river channels migrate and reincorporate such stored alluvium into bed load, there is a net increase in the proportion of sediment that has been stored--and subjected to weathering during alluvial storage. Climate change, tributary dilution, and anthropogenic perturbation of the river system are insufficient to account for these compositional changes. This process is similar to that demonstrated previously in tropical river systems. The magnitude of the compositional variation among the Sagavanirktok River sands suggests that weathering during alluvial storage is a dominant process in shaping sediment composition in Arctic settings as well. Chemical weathering in the arid, Arctic setting of the North Slope of Alaska is probably enhanced by high organic acid activity associated with the stagnant water perched on permafrost of the tundra. Physical weathering in the form of grain disaggregation through freeze-thaw cycles, crushing between cobbles, and dragging by ice probably plays an important role as well.