RIVER SYSTEMS
Earth's Dynamic Systems:  Chapter 12  (pps. 258-297)


Major Characteristics of a River Stream:

A river system consists of a main channel and all of the tributaries that flow into it.  Within a river system, the surface of the ground slopes toward the network of tributaries, so the drainage system acts as a funneling mechanism for removing surface runoff and weathered rock debris.  A typical river system can be divided into three subsystems:

See Figure 12.2 on page 262 for a tree-type depiction of the major parts of a river system.

NOTE:  Running water is by far the most important agent of erosion.  Stream valleys are the most common landforms on the continents.
 
   tributary    a stream flowing into or joining a larger stream
  distributary     any of the numerous stream branches into which a river divides where it reaches its delta
  upstream    moves toward headwater (up the regional slope of erosion)
  downstream     moves toward mouth of river (delta)
  delta    a large, roughly triangular body of sediment deposited at the mouth of a river
  meander    a broad, looping bend in a river  (e.g.  Brownsville is 20 road miles from Boca Chica; it's 120 river miles!) 
 
 

Drainage Patterns of Streams:  type of bedrock greatly influences the type of drainage pattern

A river system is bounded by a divide (ridge beyond which water is drained by another system).  The divide separates flow into stream patterns that can be on a small or large scale.
W---------------|---------------E

An example of large scale is Continental Divide -- on one side the water flows to Pacific, on the other side the water flows to Atlantic.  (Note:  Everything in Texas flows to Gulf of Mexico.)
 

Concepts Relating to Streams:   (page 262)

 


Dynamics of Stream Flow:   (page 263)

Rivers are highly complex systems influenced by a number of variables and, as is the case with so many natural systems, if one variable is changed it produces a change in the others.  The most important variables are:
 
  discharge     the amount of water passing a given point during a specific time interval; the rate of flow is measured in cu ft/sec or m/sec  Rio Grande 
  discharge in Laredo  
=  3000 cu ft/sec 
  discharge in Brownsville  
=  80 cu ft/sec 
  becomes an estuary because water is siphoned for human use
   discharge of Mississippi = about 1,000,000 cu ft/sec  

   discharge of Amazon 
(world's largest river in terms of water volume) = 
several million cu ft/sec

  velocity   speed of the flow of water in a river system   center of a straight channel is where velocity is greatest;  curved channels provide highest velocities;  narrow / shallow channels have too much friction from surface area(s)   outside (erosion side) of a meander bend is where velocity is greatest;  inside (deposition side) of a meander bend is where velocity is lowest
  gradient   the slope of the stream channel; 
also influences velocity 
  the steeper the gradient, the higher the velocity    the gradient of a stream is steepest in the headwaters and decreases downslope
 sediment load  
-- transported 3 ways
  suspended load -- mud, silt, and sand carried in suspension for considerable period of time without contact with the stream bed;  typically the largest amount of sediment carried by river   bed load -- coarse particles dragged along river bottom;  usually about 10% of river's sediment, however, percentage may rise during a flood   dissolved load -- matter transported in form of chemical ions and essentially invisible; once mineral matter is dissolved, it remains in solution, and is precipitated and deposited only if chemistry of water changes; only a very small percentage of what the river carries
  base level    lowest level to which a stream can erode its channel;  sea level (ocean level) is the ultimate base level   base level can change--sea level can drop, land can be uplifted, or land can subside   special places (Death Valley, CA) are below sea level
 

Rivers are like conveyor belts and function to transport water and sediment.  A river system functions as a unified whole:  any change in one part of the system affects the other parts.

graded stream:  a stream in equilibrium, or balance, between erosion and deposition; not cutting its channel any deeper, basically just carrying sediment
 



Processes of Stream Erosion:   (page 271)  

Processes of Stream Deposition:   (page 279)

In the lower parts of the drainage system (transporting and dispersing systems) the gradient of the rivers is very low.  As a result, the stream's velocity is reduced, and deposition of the sediment load occurs, creating distinctive landforms.  Foremost among these are:

     
  

River Systems and Plate Tectonics:   (page 292)

The evolution of the major rivers of the world is influenced directly and indirectly by plate tectonics.