Figure 2-9. Depositional features of fluvial systems
(d) Deltas. When streams carrying suspended sediments enter the sea or another standing body of
water, there is a considerable loss in flow velocity, causing a massive deposition of sediment. Naturally, the
coarser particles, such as sand and gravel, settle from suspension first, while the finer material is carried farther
out to sea. The result is the formation of a depositional feature called a delta. The shape of a delta varies
depending on the relative influences of the river, waves, and tides.
River-dominated deltas take on a lobate appearance where the sediment supply is moderate and an elongate, or
bird's-foot, appearance where there is a large sediment supply (see figure 2-10). A tide-dominated, or estuarine
delta is composed of many linear distributaries that are aligned parallel to tidal flow and therefore perpendicular
to the shore (see figure 2-10). Wave-dominated deltas tend to take on an arcuate shape (see figure 2-10). Figure
2-11 is an excellent example of an arcuate deltaic feature.
b. Fresh Water. Standing bodies of fresh water, such as lakes or ponds, are generally sites of deposition
rather than erosion. Any deposit that owes its origin to a lake is referred to as a lacustrine deposit. Lacustrine
deposits are usually made up of fine-grained material (silt or clay). In humid climates, successive growths of
vegetation sometimes fill small lakes or ponds, creating the organic lacustrine deposits of freshwater marshes,
swamps, and bogs. These deposits are characterized by flat topography, heavy vegetative growth, and highly
compressible soils with a high moisture content. They also exhibit a lack of surface drainage. In arid or semiarid
climates, dry lake-bed deposits, called playas or sebkhas, are common. These deposits are composed of
evaporites, such as salt and gypsum, in addition to the fine-grained silts and clays. Playas and sebkhas can be
recognized by their flat topography, lack of vegetation, and relationship to surrounding areas of higher elevation.