Much of the weathered debris in deserts comes from mechanical weathering like alluvial fans formed where stream slopes abruptly decrease. While chemical weathering is less common, over long periods clays and thin soils can form. Desert streams are ephemeral, only flowing after rain, and often drying up before reaching oceans due to evaporation and soil infiltration. Most desert erosion comes from running water, though wind also erodes deserts, water does the most erosional work.