Rivers and fluvial processes (chapter 16)

Chapter 16 begins with a discussion of the hydrologic cycle, the relative percentages of the earth's water are found in different storage reservoirs (e.g. ocean, glaciers, groundwater, atmosphere, etc.), and the exchange among the major reservoirs by precipitation, evaporation and transpiration (or, in combined form, evapotranspiration), infiltration, and runoff. The role of running water in shaping the earth's surface is related to the mechanisms that generate surface runoff, which results when the amount of water reaching the surface exceeds the infiltration capacity of the soil. Flow over the soil surface quickly concentrates in local low points and depressions and becomes channelized, carving rills that ultimately drain into local stream channels. The chapter continues by describing the basic characteristics of flow and sediment transport in stream channels. However this set of notes changes the sequence of topics somewhat; I prefer to start by pointing out a few essential facts about the large-scale organization of fluvial landscapes before proceeding to the fine details of flow and channel form.

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, rectangular, 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. 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. (Note: you will not find the word "watershed" in the glossary or the index of the textbook, but nowadays it is generally used interchangeably with the term "drainage basin".) 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.

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 (refer to chapter 17, figs. 17.2 and 17.5). 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). 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. 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. Furthermore there is a tendency for the hydrograph to have a higher peak and a corresponding increase in the likelihood of flooding.

With this as background, let's proceed to outline the remaining major topics to be covered. These are not necessarily in the same order as you will find in chapter 16; I suggest that you read the chapter with these notes in hand and check off the appropriate pages for each topic as you reach it.

• Physical character of flow in stream channels: laminar vs. turbulent flow and the dominant role of turbulent flow in natural channels

• Streamflow and sediment transport: note the conversion of potential energy to kinetic energy as water flows downslope; frictional drag against bed and banks diverts some of this energy for scour and transport of sediment.

• Sediment transport: suspended load and bed load, also note importance of dissolved load. Bed load travels either by rolling or by bouncing, which takes place in an intermittent series of jumps called saltation. Sediment traveling along the bed of the stream often forms dunes and ripples, which may be preserved in the geologic record as cross-bedded sandstones.

• Rise and fall of water level and of discharge during a storm event: you will find a definition of discharge on p. 472, but not a graph illustrating how discharge changes over time during a storm. This is known as a hydrograph and it will be described in class. Increasing capacity for sediment transport causes scour of the riverbed as the flow rate increases during a storm or high-flow event, and  decreasing sediment transport capacity causes deposition as the flow rate decreases. Competence (i.e. the maximum particle size that can be transported) also tends to increase at higher flow and decreases again as flow declines. This occurs because the settling velocity of sediment is a function of particle size: the larger the particle, the faster it settles and the swifter the current that is needed to keep it from settling. Capacity - the maximum total amount of sediment that can be transported at any moment - also increases with discharge and decreases as the flow rate wanes after the peak of the hydrograph. In fact very little sediment is carried during low-flow conditions in most rivers, whereas most of the sediment transported in any given year will probably be transported in just a few high-flow events or floods.

• Streams are able not only to carry loose sediment, but also to erode bedrock. This is accomplished by abrasion, which is the process that results when sediment particles being carried byt the flow are used as tools to scrape and grind the rock surface. An eroding bedrock surface often is pockmarked by a series of potholes.

• The typical pattern of erosion in steep terrain is for the stream to cut a v-shaped notch in the landscape.   However many stream valleys have flat or nearly flat valley floors bounded by steeper hillslopes and upland areas. Often the stream channel is bordered by a floodplain, which is itself created by deposition of  alluvial sediment on the valley floor. Although it may be above water most of the time, the floodplain is flooded at least occasionally. (In fact, with an adequate record of past flows we can make approximate predictions of the likelihood that a flood of a given size will occur in any given year.) When water flows over the banks and spreads out,  it slows down and  some of the sediment in transport drops out very rapidly. This can lead to formation of natural levees bordering the channel.

• The remnants of former floodplains, left behind as the channel cuts downward (possibly from uplift of the landscape, but with several other possible causes), are referred to as terraces.

 
• Velocity distribution in a typical channel: maximum just below the surface over the deepest part of the channel, where it is least affected by frictional loss of energy along the bed. In a meandering river, the zone of greatest depth and maximum velocity shifts back and forth from one side of the channel to the other, but always tends to be located along the outer or concave bank in a meander bend.

• Channel form: "typical" varieties of plan form or channel pattern
• straight (relatively rare)
• meandering
• pattern of flow in a meandering channel: main thread of flow shifts from side to side, maximum velocity on outside of bend
• deposition and formation of point bar on convex (inner) bank, erosion of concave (outer) bank (cut bank), lateral migration of meanders, formation of meander cutoffs, oxbow lakes, back swamps, natural levees, etc.
• braided
• highly variable discharge, erodible banks, excess sediment load, unstable individual channels with rapid deposition of bars and diversion of flow
• variants on major themes (e.g. mixed channel types, stable anastomosing or anabranching channels, etc.; these are interesting but don't worry about them if we don't get to them in class.)
• downcutting may lead to abandonment of floodplain, forming terraces, or even to vertical incision of the meanders themselves into bedrock (incised meanders, see for example p.492)
  
  
• Longitudinal trends from upstream to downstream  in a watershed:
• streams typically get larger, as they have to carry more water with increasing drainage area (this may not be true in desert areas, however); typically, therefore, width, depth, and velocity all will increase downstream with increasing discharge as water is collected from larger drainage areas. This may seem counterintuitive as the more turbulent flow in small headwater channels may look faster; but in fact velocity does increase as you go downstream
• gradient or slope generally becomes gentler in the downstream direction (this may be interrupted by knickpoints, waterfalls, or local changes in bedrock resistance that create steeper channel gradients); typical longitudinal profile is a concave-upward curve
• caliber or particle size of sediment generally decreases downstream (except at locations where fresh inputs of coarse sediment may occur from local sources); coarsest sediment is typically in the steep headwater channels
• streams give the appearance of being adjusted to the base level at the downstream end, which may be sea level or a natural or man-made lake
• adjustments of base level may adjustments elsewhere along the profile:
• lowering of base level causes a steepening of the profile and incision or downcutting
• raising of base level creates a gentler gradient upstream, possibly causing deposition and raising the bed level
• a graded stream is described as one whose gradient and other geometric characteristics are adjusted so thtat the stream can just carry the sediment supplied to it, without either eroding or depositing a significant amount of sediment
  
  
• Sharp breaks in slope cause formation of depositional land forms:
• alluvial fans form along mountain fronts where streams emerging from the mountains abruptly drop their coarse sediment load; rapid back and forth switching causes formation of cone-shaped deposit. These fans are sometimes similar in shape to, but are much steeper and coarse than deltas
• deltas form where rivers enter lakes or coastal waters and drop their sediment load as they reach sea level; progressive seaward or lakeward growth actually extends the length of the stream and results in a gentler gradient
• like drainage networks in reverse, deltas split into multiple distributaries with a series of other diagnostic features related to the pattern of sediment deposition
• as distributaries grow longer and gradients along the main stream gecome gentler, chances increase that the flow will take a shorter, steeper path to the outlet and abandon that part of the delta (see, for example the discussion of the Mississippi Delta)
• The chapter also includes a brief section on floods and flood control