Fluvial deposits are an important part of the ancient sedimentary record. The reason why is not obvious; after all, rivers drain areas of the continents that are undergoing erosion. Most rivers, except the smallest, are alluvial rivers: they have a bed, and a floodplain, composed of their own sediments. But in most cases this alluvial valley sediment is not very thick. Only in certain cases does the alluvial valley fill become thicker.
Two effects are conducive to deposition in rivers: progradation and crustal subsidence.
Progradation. As a river wears down the land and delivers sediment to the sea, the mouth of the river builds seaward. Because the longitudinal profile of a river is anchored by base level at the mouth, this means that there has to be a slight upbuilding in the lowermost reach of the river (Figure 3-47). This may not seem like a big effect, but even some tens of meters is a lot of sediment.
Crustal subsidence. The only way to get a really thick sequence of fluvial sediment is to drop the continental crust beneath the river. As this happens, slowly, along some reach of the river, there develops a very slight expansion of flow and decrease in flow velocity and therefore in sediment-moving ability. Just by simple bookkeeping, this must lead to storage of sediment along the river: if what comes into a given area of the bed is greater than what goes out, sediment is stored in that area, and the bed builds up. From an anthropomorphic standpoint, the river tries to maintain its longitudinal profile while the bottom drops out from under it, and it does so by leaving a little of the passing sediment to build up its bed (Figure 3-48).