Subscribe to our newsletter. This top image shows part of an alluvial terrace in the southern Sierra Nevada, where boulders as large as 1 meter mix with sand and gravel, washed down a side canyon into the Kern River valley. The Kern has undercut the terrace here and carried away part of the deposit, starting a new cycle of transport and deposition somewhere downstream.
Alluvial sediment is pounded and ground into finer and finer grains each time it moves downstream.
The process can take thousands of years. The photo below is from a small alluvial fan at the foot of the Temblor Range in west-central California. Here the alluvium is all of gravel size and smaller. Each of the visible layers in this streambank exposure was probably laid down in a single flood event. The feldspar and quartz minerals in alluvium weather slowly into clays and dissolved silica.