River systems, fluvial processes and landscapes, floodplains, and river control strategies are important to human populations as demands for limited water resources increase. Stream-related processes are called fluvial (from the Latin word fluvius = river).
Water dislodges, dissolves, or removes surface material in the process called erosion. Streams produce fluvial erosion, in which weathered sediment is picked up for transport, and movement to new locations. Sediments are laid down by another process, deposition. Alluvium is the general term for the clay, silt, and sand deposited by running water.
The basic fluvial system is a drainage basin, the spatial geomorphic area occupied by a river system. The drainage basis is an open system. Drainage divides define the watershed catchment (water receiving) area of the drainage basin. In any drainage basin, water initially moves downslope in a thin film called sheetflow, or overland flow. This surface runoff concentrates in rills, or small-scale downhill grooves, which may develop into deeper gullies and a stream course in a valley.
High ground that separates one valley from another and directs sheetflow is termed an interfluve. Extensive mountain and highland regions act as continental divides that separate major drainage basins. Drainage pattern refers to the arrangement of channels in an area as determined by the steepness, variable rock resistance, variable climate, hydrology, relief of the land, and structural controls imposed by the landscape. There are seven basic drainage patterns generally found in nature: 1. dendritic, 2. trellis, 3. radial, 4. parallel, 5. rectangular, 6. annular, and 7. deranged.
Stream channels vary in width and depth. The streams that flow in them vary in velocity and in the sediment load they carry. All of these factors may increase with increasing discharge. Discharge is calculated by multiplying the velocity of the stream by its width and depth for a specific cross section of the channel.
RIVER EROSION AND TRANSPORTATION: kinds of Processes:
a) Hydraulic action is the work of turbulence in the water. Running water causes hydraulic squeeze-and-release action to loosen and lift rocks and sediment. As this debris moves along, the river mechanically erodes the streambed further with the load it is carrying through a process of b) abrasion. c) 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-grained, clastic particles held aloft in the stream, with the finest particles held in suspension until the stream velocity slows nearly to zero. Bed load refers to coarser materials that are dragged along the stream bed by 1) traction or are rolled and bounced along by 2) saltation. If the load in a stream exceeds its capacity, aggradation occurs, or the accumulation of excess sediment, as deposition fills the stream channel. With excess sediment, a stream becomes a maze of interconnected channels that form a braided stream pattern.
FLOW CHANNEL CHARACTERISTICS
Where the slope is gradual, stream channels develop a sinuous form called a meandering stream. The outer portion of each meandering curve is subject to the fastest water velocity and can be the site of a steep undercut bank. On the other hand, the inner portion of a meander experiences the slowest water velocity and forms a point bar deposit. When a meander neck is cut off as two undercut banks merge, the meander becomes isolated and forms an oxbow lake.
Every stream develops its own gradient and establishes a longitudinal profile. A portion of the stream is designated a graded stream when the stream is adjusted among available discharge, channel characteristics, its velocity, and the load supplied from the drainage basin. An interruption in a stream’s longitudinal profile is called a nickpoint. A nickpoint can develop as the stream flows across hard resistant rock or after tectonic uplift episodes.
Floodplains have been an important site of human activity throughout history. Rich soils, bathed in fresh nutrients by floodwaters, attract agricultural activity and urbanization. Despite our knowledge of historical devastation by floods, floodplains are settled, raising issues of human hazard perception. The flat, low-lying area along a stream channel that is subjected to recurrent flooding is a floodplain. It is formed when the river overflows its channel during times of high flow. On either bank of most streams, natural levees develop as by-products of flooding. During floods when the river overflows its banks, it loses velocity as it spreads out and drops a portion of its sediment load to form levees. On the floodplain, backswamps and yazoo tributaries may develop. Alluvial terraces are giant steps on each side of a river formed by the entrenchment of a river into its own floodplain.
A depositional plain formed at the mouth of a river is called a delta. Each flood stage deposits a new layer of alluvium at the mouth of the river sometimes choking the river flow. This forces the river to break up into several distributaries. There are several kinds of deltas:
a) The Nile river (Africa) and Danube river (Europe) have an Arcuate Delta (arc-shaped).
b) The Seine river (France) has an Estuarine Delta, seaward mouth of a delta. When the mouth of a river enters the sea and is inundated by the sea in a mix with freshwater and very little delta, it is called an estuary.
c) The Mississippi rive has a bird-foot delta. A long channel with many distributaries and sediments carried beyond the tip of the delta into the sea.
FLOODS AND RIVER MANAGEMENT
A flood occurs when high water overflows the natural or artificial levees of a stream and spreads out into its flood plain. Both floods and the floodplains they might occupy are rated statistically for the expected time interval between floods. A 10-year flood is the greatest level of flooding that is likely once every 10 years. A graph of stream discharge over time for a specific place is called a hydrograph. Collective efforts by government agencies undertake to reduce flood probability. Such management attempts include the construction of artificial levees, bypasses, straightened channels, diversions, dams, and reservoirs. Society still is learning how to live in a sustainable way with Earth’s dynamic river systems.
1. What role is played by rivers in the hydrologic cycle?
2. What are the five largest rivers on Earth in terms of discharge? Relate these to the weather patterns in each area and to regional potential evapotranspiration (moisture demand) and precipitation (moisture supply).
3. Define fluvial. What is a fluvial process?
4. What is the sequence of events that takes place as a stream dislodges material?
5. According to Figure 11-3, in which drainage basin are you located? Where are you in relation to the various continental divides?
6. What is the spatial geomorphic unit of an individual river system? How is it determined on the landscape? Define the several relevant key terms used.
7. On Figure 11-3, follow the Allegheny-Ohio-Mississippi river systems to the Gulf of Mexico, analyze the pattern of tributaries, and describe the channel. What role do continental divides play in this drainage?
8. Describe drainage patterns. Define the various patterns that commonly appear in nature. What drainage patterns exist in your hometown? Where you attend school?
9. What was the impact of flood discharge on the channel of the San Juan River near Bluff, Utah? Why did these changes take place?
10. How does stream discharge complete its erosive work? What are the processes at work in the channel?
11. Differentiate between stream competence and stream capacity.
12. How does a stream transport its sediment load? What processes are at work?
13. Describe the flow characteristics of a meandering stream. What is the pattern of flow in the channel? What are the erosional and depositional features and the typical landforms created?
14. Explain these statements: (a) All streams have a gradient, but not all streams are graded. (b) Graded streams may have ungraded segments.
15. Why is Niagara Falls an example of a nickpoint? Without human intervention, what do you think would eventually take place at Niagara Falls?
16. Apply these concepts (gradient, graded stream, meandering stream, nickpoint), where appropriate to a stream in your area. Explain and discuss.
17. Describe the formation of a floodplain. How are natural levees, oxbow lakes, backswamps, and yazoo tributaries produced?
18. How is it possible to travel fewer kilometers on the Mississippi River between St. Louis and New Orleans today than 100 years ago? Explain.
19. Describe any floodplains near where you live or where you go to college. Have you seen any of the floodplain features discussed in this chapter? If so, which ones?
20. What is a river delta? What are the various deltaic forms? Give some examples.
21. How might life in New Orleans change in the next century? Explain.
22. Describe the Ganges River delta. What factors upstream explain its form and pattern? Assess the consequences of settlement on this delta.
23. What is meant by the statement, “the Nile Delta is receding” (caption Figure 11-21)?
24. Specifically, what is a flood? How are such flows measured and tracked?
25. Differentiate between a hydrograph from a natural terrain and one from an urbanized area.
26. What do you see as the major consideration regarding floodplain management? How would you describe the general attitude of society toward natural hazards and disasters?
27. What do you think the author of the article “Settlement Control Beats Flood Control” meant by the title? Explain your answer, using information presented in the chapter.