“When a mountain stream carries a lot of sediment (clay, silt, sand, gravel, cobbles, and boulders) and leaves the conﬁnes of the canyon, the sediment is deposited. Over time, this process creates a fan-shaped deposit. The sediment is deposited not because the gradient of the stream decreased, but because the power of the stream dissipates beyond the canyon mouth as the water is spread thin and inﬁitrates. Many cities are built on alluvium fans, often leading to hazards from ﬂash ﬂoods and mudflows.”
In mountainous, interior deserts, the largest settlements are usually not deep in the deserts or on top of the mountains but in that in between space. Many settlements in Central Asia are built on these alluvial fans.