Figures 3-17 and 3-18 are simplified flow-transverse cross sections through a representative single-channel alluvial river of medium to large size. Figure 3-17 shows the entire stream valley, and Figure 3-18 shows details of the river channel itself. In the following paragraphs I will elaborate upon the various features shown in these figures the diagrams. I’ll defer a description of the planform features of the river (that is, what you would see from the air, above the river) until later.
Most medium to large rivers flow on beds of sediment that they have deposited and can transport again; in a later section of this chapter, rivers of this kind will be called alluvial rivers. The unconsolidated sediment in the river valley, lying above the bedrock “basement” of the river, is called the valley fill. Its thickness ranges from just a veneer to hundreds or even thousands of meters. In the case of rivers flowing across areas of the crust that have undergone substantial and prolonged subsidence, the valley fill is buried so deeply that it is at least partly lithified, and the material grades over into what would be considered the “ancient sedimentary record” (the term geologists use for sedimentary rocks that are very old by human standards).
The floodplain of a river is an area of low relief adjacent to the river channel, which is inundated at times of high river stage. During floods the floodplain receives a layer of fine sediment that settles out of suspension as the flood waters spread over the floodplain and decrease in velocity. If the river is not undergoing net aggradation (see below for what I mean by that), then the floodplain builds up to a level at which the rate of removal of fine sediment by erosion back into the main channel at times of low water is great enough to strike a balance with the rate of addition of fine sediment from suspension during floods. Most river floodplains are heavily vegetated, and, depending upon climate, are often dotted with shallow lakes and swamps (called backswamps). Floodplains are among the best areas for agriculture, because they continually receive fresh influxes of fertile soil.
Alongside many river channels are low ridges called natural levees, formed by deposition of the finer fraction of suspended sediment from flood waters passing across the river banks when the river is above flood stage. There’s preferential deposition because the flood waters decelerate as they leave the main channel flow.
The river channel itself can be characterized most fundamentally by its cross-section shape and cross-section area. The width is the distance, normal to the local trend of the river, from bank to bank; obviously the width depends strongly on the river stage as well as on the average size of the river. The depth of the river varies from point to point across the section. A good way of encapsulating the lateral dimensions of the river is to specify the hydraulic radius: the ratio of the cross-sectional area to the wetted perimeter at a given cross section. (To figure out the wetted perimeter, you would use one of those distance- measuring wheels you can rent or buy. Start at the water line on one bank and walk straight across the river to the water line on the opposite bank. Whether you could do that without underwater breathing gear depends on the depth of the river.) For a very wide channel with a nearly rectangular cross section, with an approximately level bottom and steep banks, the hydraulic radius is nearly equal to the flow depth. (You might try figuring that out for yourself; ,it would take some careful thought and a bit of math.)
Another significant aspect of river geometry is the vertical profile. Imagine traveling up the river, keeping track of two things: the elevation of the riverbed above sea level, and the map distance from the mouth of the river. Then plot a graph with the riverbed elevation on the vertical axis and the upstream distance on the horizontal axis. Pass a smooth curve through the points. The result is what is called the longitudinal profile (or long profile) of the river.
The longitudinal profiles of most rivers are concave upward, as shown in Figure 3-19. The reason is not difficult to understand. In the downstream direction, one tributary after another joins the river, each adding discharge. As the river grows larger, the ratio of cross-sectional area to wetted perimeter increases. Because the slope of the river depends, in large part, on the relative magnitude of the downslope driving force of gravity, which is affected by the whole volume of the river, and the upslope resisting force of friction, which is affected by the area of the riverbed, the slope decreases downstream.
The base level of a river is the elevation of the water surface of the water body, either the world ocean or a lake along the river course, into which the river flows (Figure 3-20). The base level changes with time: lake levels fluctuate as a consequence of variations in precipitation in the watershed of the river or because the outlet of the lake is eroded downward, and sea level changes, for various reasons and often very substantially, over a great variety of time scales, ranging from decades to tens of millions of years.
Think about what happens to the river as its base level changes. The concept to keep mind is that the river has some equilibrium longitudinal profile, in the sense that if conditions of precipitation, sediment supply, and base level remain constant the longitudinal profile stays the same. If a different set of conditions is imposed upon the river, the river adjusts its longitudinal profile accordingly toward a new equilibrium.
If base level rises, some of the sediment that’s carried along by the river toward the river mouth is deposited along the way to raise the river bed, thereby establishing a new equilibrium longitudinal profile. If base level falls, the river erodes its bed to adjust toward a new, lower equilibrium profile.
There’s more to be said, however, about what happens as the river erodes its bed as a consequence of a fall in base level. The erosion does not happen uniformly everywhere. all at the same time, but by upstream propagation of a point where the channel slope changes, from steeper downstream of the point to less steep upstream of the point. The point of change in slope is called a knickpoint (Figure 3-21). The position of a knickpoint is marked by a waterfall or rapids. Knickpoints migrate slowly upstream, thereby extending the new, lower longitudinal profile as the river eats its way upstream. If a floodplain has developed in the river valley, the old floodplain downstream of the knickpoint survives, for a long time, as a pair of terraces above the new, lower river channel Because the difference between old and new equilibrium profiles decreases upstream, other things being equal (the elevations of the highlands in the headwaters of the river are very conservative), the height of this knickpoint decreases as it migrates upstream. Often, if base level drops abruptly a number of times during some long period of time, more than one knickpoint is present along the river course, each slowly making its way upstream.
As you will see in the later material on the plan-view features of rivers, rivers do not stay in one position but instead tend to shift laterally across their floodplains, by erosion at one bank and deposition at the other bank. (That’s the basic reason why there are floodplains in the first place.) As the river lowers its bed in response to a fall in base level, and at the same time shifts its course laterally, it develops a new floodplain that’s entrenched below the level of the old floodplain. The result is a pair of flat-topped river terraces, one on either side of the river. The slopes at the edges of the modern floodplain retreat without much change in their shape, because they are continually being undercut along their bases rather than wearing away over their entire surface. Sometimes there is more than one set of such terraces.