Figure 2-3. Comparison of coarse-textured and fine-textured drainage
Finer-grained rocks, such as shales, are fairly impermeable. Consequently, surface runoff is at a maximum, and
erosion is often intense. The very dense drainage systems that result from this type of situation are called fine-
textured drainage systems (see figure 2-3). In order to compare the densities of two or more drainage systems,
those systems must be displayed at the same scale. Drainage textures are most frequently based on their
appearances at scales of approximately 1:20,000.
(c) Other Erosional Features of Fluvial Systems. Streams rarely exhibit a straight-line flow;
rather, they are gradually deflected from alignment by slight irregularities in their courses. The loops that form as
a result of these deflections are called meanders, and the streams are called meandering streams. As channelized
water flows downstream and rounds these loops or bends, pressure is concentrated against the channel wall on the
outside of the curve, causing its erosion (deposition, which will be discussed later, occurs on the inside of the
curve). As erosion progresses, the stream migrates, leaving abandoned channels and oxbow lakes in its wake. In
addition to lateral erosion, the stream may also exhibit downward erosion. In these cases, old, level floodplains
may be left high and dry while new ones are formed at lower elevations. Erosional slopes called alluvial terraces
exist between the various levels of floodplains, producing a stair-step type topography. Figure 2-4 depicts several
major erosional features associated with fluvial systems.
(d) The Fluvial Cycle of Erosion. There are three different stages that may be observed in the
evolution of a stream-youth, maturity, and old age.