Geography 300-Chapter 11

What are the five largest rivers on Earth in terms of discharge?

...

Differentiate between stream competence and stream capacity.

Competence, which is a stream's ability to move particles of specific size, is a function of stream velocity. The total possible load that a stream can transport is its capacity.

How does a stream transport its sediment load? What processes are at work?

Four processes transport eroded materials: solution, suspension, saltation, and traction. Solution refers to the dissolved load of a stream, especially the chemical solution derived from minerals such as limestone or dolomite or from soluble salts. The suspended load consists of fine particles physically held aloft in the stream, with the finest particles not deposited until the stream velocity slows to near zero. The bed load refers to those coarser materials that are dragged along the bed of the stream by traction or are rolled and bounced along by saltation (from the Latin saltim, which means "by leaps or jumps."

Differentiate between a natural stream hydrography and one from an urbanized area

A graph of stream discharge over a time period for a specific place is called a hydrograph. The hydrograph in Figure 14-29a (next slide), shows the relationship between stream discharge and precipitation input. During dry periods, at low water stages, the flow is described as base flow and is largely maintained by contributions from the local water table. When rainfall occurs in some portion of the watershed, the runoff collects and is concentrated in streams and tributaries. The amount, location, and duration of the rainfall episode determine the peak flow. Also important is the nature of the surface in a watershed; for example, a hydrograph for a specific portion of a stream changes after a forest fire or urbanization of the watershed.

What is the sequence of events that takes place as a stream dislodges material?

...

What was the impact of flood discharge on the channel of the San Juan River near Bluff, UT? Why did these changes take place?

...

Describe the flow characteristics of a meandering stream. What is the pattern of the flow in the channel? What are the erosional and depositional features and the typical landforms created?

A meandering channel pattern is common for a stream that slopes gradually, a sinuous (wavy) form weaving across the landscape. The outer portion of each meandering curve is subject to the greatest erosive action and can be the site of a steep bank called a cut bank (Figures 14-14 in next slide). On the other hand, the inner portion of a meander receives sediment fill and forms a deposit called a point bar. As meanders develop, these scour-and-fill features gradually work their way downstream. If the load in a stream exceeds the capacity of the stream, sediments accumulate in the stream channel as the channel builds up through deposition. With excess sediment, a stream becomes a maze of interconnected channels laced with sediments that form a braided (mixing) pattern.

How does stream discharge do its erosive work? What are the processes at work on the channel?

Several types of erosional processes are operative. Hydraulic action is the work of turbulence in the water—the eddies of motion. Running water causes friction in the joints of the rocks in a stream channel. A hydraulic squeeze-and-release action works to loosen and lift rocks. As this debris moves along, it mechanically erodes the streambed further through the process of abrasion, with rock particles grinding and carving the streambed.

What is the spatial geomorphic unit of an individual river system? How is it determined on the landscape? Define the key relevant terms used

Streams are organized into areas or regions called drainage basins. A drainage basin is the spatial geomorphic unit occupied by a river system. A drainage basin is defined by ridges that form drainage divides, i.e., the ridges are the dividing lines that control into which basin precipitation drains. Drainage divides define watersheds, the catchment areas of the drainage basin (see next two slides). The United States and Canada are divided by several continental divides; these are extensive mountain and highland regions that separate drainage basins, sending flows either to the Pacific, or to the Gulf of Mexico and the Atlantic, or to Hudson Bay and the Arctic Ocean.

What does "Settlement Control Beats Flood Control" means?

There are other ways to protect populations than with enormous, expensive, sometimes environmentally disruptive projects. Strictly zoning the floodplain is one approach, (but flat, easily developed floodplains near pleasant rivers might be perceived as desirable for housing, and thus weaken political resolve). This strategy would set aside the floodplain for farming or passive recreation, such as a park, golf course, or plant and wildlife sanctuary, or for other uses which are not hurt by natural floods.

Describe drainage patterns. Define the various patterns that commonly appear in nature.

A drainage basin is the spatial geomorphic unit occupied by a river system. A drainage basin is defined by ridges that form drainage divides, i.e., the ridges are the dividing lines that control into which basin precipitation drains. Drainage basins are open systems whose inputs include precipitation, the minerals and rocks of the regional geology, and both the uplift and subsidence provided by tectonic activities. System outputs of water and sediment leave through the mouth of the river. Change that occurs in any portion of a drainage basin can affect the entire system as the stream adjusts to carry the appropriate load relative to discharge and velocity.

What is a flood? How are such flows measured and tracked?

A flood is a high water level that overflows the natural (or artificial) banks along any portion of a stream. Understanding flood patterns for a drainage basin is as complex as understanding the weather, for floods and weather are equally variable, and both include a level of unpredictability. The key is to measure streamflow—the height and discharge of a stream. A staff gauge, a pole placed in a stream bank and marked with water heights, is used to measure stream level. With a fully measured cross section, stream level can be used to determine discharge. A stilling well is sited on the stream bank and a gauge is mounted in it to measure stream level. A portable current meter can be used to sample velocity at various locations.

Explain the base-level concept. What happens to a local base level when a reservoir is constructed?

Base level is a level below which a stream cannot erode its valley further (see Figure 14-3 in next slide). The hypothetical absolute or ultimate base level is sea level (which is the average level between high and low tides). You can imagine base level as a surface extending inland from sea level, inclined gently upward, under the continents. Ideally, this is the lowest practical level for all denudation process. Although base level is a very useful concept, no satisfactory working definition has yet been agreed upon. A local, or temporary, base level may control a regional landscape and the lower limit of local streams. That local base level might be a river, a lake, a hard and resistant rock structure, or a human-made dam. In arid landscapes, with their intermittent precipitation, valleys, plains, or other low points provide local control. Reservoir and dam structure interrupt the gradient of a stream, producing a local base level that controls the upstream behavior and profile of the stream. The top of the dam is the precise location of the local base level. The load carried by the stream is deposited in the reservoir, since the stream loses velocity as it enters the body of water. If the dam should break, the stream would rapidly scour a channel through these deposits in response to a new downstream base level, forming terraces on either side of the stream through the former reservoir.