Rivers and fluvial processes (chapter 14)

NOTE: The sequence of topics in these notes does not follow the exact order of the chapter in the textbook. However you should use these notes as your guide to reading the chapter as they more closely reflect the way I approach this subject in class. I recommend that you read the chapter with these notes in hand, making sure that you check off the topics listed here and make a note of the appropriate pages numbers as you get to them in the chapter.

Rivers function as the plumbing system for the surface portion of the landscape, carrying water, dissolved and solid materials downstream under the influence of gravity. Flowing water acts as a transporting agent and as a sculpturing agent, dissecting the landscape. Because running water exerts frictional energy against the bed and banks, it has the capacity to erode, to transport sediment, and to maintain channels.

The result of fluvial erosion is a drainage network of channels adjusted to carry the water and sediment running off the land surface. Drainage network patterns reflect the influence of geologic structure (e.g., dendritic, trellis, radial, etc.)

The watershed or drainage basin is that area of the landscape drained by a particular river or stream and its tributaries;  the drainage divide is defined topographically as the boundary around that area.  Virtually the entire surface area of the continents can be subdivided into a series of watersheds. Each watershed has a nested structure: in other words, wherever two streams join, the area drained by the stream below the junction includes the total area of both watersheds. This surface area is referred to as the drainage area of the watershed. Drainage density tells us about the total length of all streams in a watershed per unit of drainage area. The higher the drainage density, the more efficiently the surface drainage network carries water off the land surface. We can recognize watersheds that are as small as the area contributing to a channel draining a single hillside, or as large as the Mississippi or even the Amazon watershed.  In a sense the watershed is the most natural unit for subdividing the landscape, since everything that happens within the watershed boundary has the potential to affect conditions downstream along the river. For that reason, we often find environmental impact statements and ecosystem studies organized by watershed.

The size of a stream channel typically increases in the downstream direction with increasing drainage area and increasing flow provided from the upstream watershed.

Where does streamflow come from?

Stormflow - surface runoff that is generated during a storm by rain on saturated areas, or by overland flow from areas where the rain falls faster than the water can seep into the ground. This surface flow eventually finds its way to a stream channel, and the network of channels concentrates flow downstream as the area contributing runoff increases.

Baseflow - seepage from the intersection of the water table with a stream channel. Water flowing in a stream during a period of no precipitation is baseflow, and is generally derived by seepage of groundwater into the bed of the stream.

In either case we measure streamflow in volume per unit time, i.e. cubic feet per second or cubic meters per second. It can becalculated as the product of (depth x width x average velocity of flow) or Q = wdv. The term for this measured quantity is discharge, and it is equivalent in meaning to the flow rate from a faucet or the pumping rate from a well (e.g. flow rate in gallons per minute). During a storm, there is typically a much larger volume of water contributed from stormflow than from baseflow. We can draw a graph of the rate of discharge as it changes through time, and we tend to see a dramatic rise during a storm, reaching a maximum or peak value before it begins to subside again. The graph we have just described is called a hydrograph. It is also the case that as water level in a stream rises during a storm, width, depth and velocity all increase as illustrated in figure 14.9 (note that the axes of this plot are logarithmic, i.e. increasing in powers of ten.) One of the most dramatic effects of urban development is to increase the amount of paved area in a watershed. Since there is so much more water running off the landscape instead of infiltrating into the ground, this has the result of causing a larger total volume of runoff from a given storm than might have occurred under natural conditions. The water also arrives in the stream more rapidly because overland flow over a paved surface is much faster than over a vegetated surface, and urban watersheds have storm drains that intercept overland flow and deliver it directly to the stream channel. Because there is a tendency for the hydrograph to have a higher peak and a shorter time base, there is a corresponding increase in the likelihood of flooding. (The discussion of urban hydrographs is at the end of the chapter.)

With this as background, let's proceed to outline the remaining major topics to be covered: