Degrees of rounding are estimated intuitively. Because ice and freezing play a role only in northern environs, and because wind is both a minor agent and then a factor only in arid lands, they will not be covered here. Alluvial Deposits are found in two forms, plains and fans . Cross-bedding is common and dips downstream in straight reaches and toward the outside of bends. Gravels in straight reaches are imbricated upstream and toward the bends.
Levee deposits are typically sandy and outlying floodplain deposits are usually silty.
Both kinds of overbank deposits are commonly rippled in arid regions but may be structureless in temperate regions due to depostion among plants or to later bioturbation. Clay, organic muck, and peat are deposited in abandoned channels and other low parts of floodplains. Rivers that aggrade their valleys slowly also tend to show a number of fining-upward deposits. In contrast, rapid aggradation results in braided channels that together cover the entire valley floor, constructing broad sand or gravel deposits characterized by closely spaced sets of trough cross-strata. In many deposits, stacked channel fills are the most clearly visible structure.