STREAMS

Hydrology or geo-hydrology is the study of surface and subsurface water--this photo is an example of a dendritic drainage pattern of streams--stream and ground waters are important sources of our useable water

I. Basic nature of streams

A. Definition, origin and course of stream

1. Definition and explanation

· a stream is a flowing body of water mostly contained in a channel at the Earth's surface--running water is a powerful agent of erosion--the water in steam channels may flow year-round (perennial streams) or may flow at intermittent times (intermittent streams) of the year based on water supply

2. Origin

· the prime source of stream water is from runoff--to understand this better we should look at the Earth's water balance called the "hydrologic cycle"---the cycle explains the fate of or paths taken by precipitation---precipitation amounts can evaporate (evaporation), be used by plants (transpiration), migrate into the subsurface (infiltration) or empty (runoff) into the lakes, ponds or streams

· click here to see the hydrologic cycle

· click here to see another version of the hydrologic cycle

3. Stream course

· the path of a stream can depend on slope of the land, geologic structures as fractures and faults and rock hardness

· the retention of water in the stream channel depends largely on the coating of channel with silt, clay or other impermeable substances

B. Stream profile and related terms

1. Stream profile

· includes the point of origin of the stream called the head, the point of termination called the mouth, and a decreasing gradient of the stream channel towards the mouth---examples of the mouth of the stream are the juncture of the stream and: another stream; a pond or lake; the ocean

· the slope of the land and stream channel is greatest at the head and smallest at the mouth— stream water flows faster if gradient or slope is higher and erodes in its channel primarily in a downward direction, while water moves slower if slope is more gentle and water may erode less downward and more sideways—if land and channel uplifts, water moves faster and erodes downward

· click here to see the stream profile

2. Base level

· is the lowest level to which a stream can erode---the ultimate base level is sea level--local or temporary base levels include lakes, resistant rock formations and main streams which act as base levels for their tributaries

3. Head-ward erosion and stream piracy

· head-ward erosion is the extension of a stream channel head-ward up the slope of erosion

· stream piracy is the diversion of the waters of one stream by another and is caused by the extension of the channel of the pirating stream by head-ward erosion—stream piracy and changing channel direction can cause large problems at local, state-national and international levels---Rio Grand River

C. Stream system

1. Tributaries

· are the smaller streams which collect materials and supply it and water to the main stream in the area

2. Main trunk

· is the main stream which receives materials from the tributaries and is the prime transporter of materials in this system

D. Stream water velocity

governs both erosion and depositional rates--generally the magnitude of erosion is proportionate to stream velocity and that for deposition is inversely proportional to velocity
Some factors affecting stream water velocity:

1. Shape and roughness of stream channel--a deep narrow channel or a wide shallow channel retard total water velocity (surface area over which the water flows is large causing more friction and slowing the water)--while a semicircular shaped channel (less surface area) offers least resistance to total water velocity click here to see the influence of channel shape on velocity--and a rough channel with boulders and large particles offers resistance to total water flowage
2. Stream gradient-- is the slope of the stream channel or number of feet dropped per mile as measured between two points along the path of the stream
3. Change in channel area along the stream--if channel area increases, the velocity of stream water decreases--alluvial fans or deltas form when depositional rates of materials increase as water velocity decreases---alluvial fans and deltas are discussed below

E. Stream loads

materials carried by stream waters--stream competence refers to the maximum size particle (according to Wentworth's table of sediment particle size) a stream can transport

1. Suspended load

· particles suspended in steam waters--usually includes clay and silt sizes--most streams carry the largest part of their load in suspension

2. Bed load

· particles bounced or skipped (saltation) along the bottom of the stream bed--usually consists of sand size particles

3. Dissolved or solution load

· the velocity of the stream has essentially no affect on a stream's ability to carry its dissolved load--precipitation occurs only when the chemistry of the water changes

II. Stream and drainage patterns, drainage basins and divides

A. Stream patterns

are in reference to individual streams

1. Straight

· a stream channel is often straight near the headwaters where much of the erosion is in a downward direction

2. Meander

· is a stream channel which meanders or is snakelike in shape--this pattern usually is more abundant farther downstream nearer the mouth of the stream--we will discuss this type of pattern in more detail later under "Stream features"

3. Braided

· is a stream with numerous inter-twinning channels--often results when the slope along the stream decreases or if the discharge of the stream decreases

· click here to see a braided stream ---- see page 476 in text for examples of stream patterns

B. Drainage patterns

are in reference to a stream and its tributaries

1. Dendritic or arborescent

· is the most common type of pattern which resembles the branching of a deciduous tree or veinlet pattern on an oak leaf

· this type of pattern forms where the rock over which the stream flows is uniform in hardness or uniform in resistance to erosion

· click here to see a photo of a dendritic drainage pattern

2. Radial

· is a pattern which forms where streams diverge from a central area like spokes from the hub of a wheel

· this type develops on volcanic cones or domal uplifts

3. Rectangular

· is a pattern which is comprised of many right-angle bends

· this pattern develops in bedrock crisscrossed by a series of joints and/or faults--the cracked surface represents precut channels in which water can flow

4. Trellis

· is a rectangular type of pattern in which tributary streams are nearly parallel to each other and have an appearance of a garden trellis

· this pattern forms in areas underlain by folded rocks resulting in alternating parallel bands of resistant and less resistant rock--best example is in the folded Appalachian Mountains

click here to see and review the drainage patterns discussed above --- see page 469 of text for drainage patterns

C. Drainage basin and divide

a drainage basin is the total land area that contributes water to a stream--the drainage basin of one stream is separated from another by an elevated area as a ridge called a divide
divides can range in size from a ridge separating two small gullies to continental divides which split continents into large drainage basins
click here to see the drainage basin and divides
the Mississippi River Drainage Basin comprises the area between the Rocky Mountain and Appalachian Mountain Divides
click here to see the Mississippi River Drainage Basin –see page 469 for another example of the US Drainage Basins

III. Stream features and channelization

A. Meander and oxbow

as mentioned before, a meander is a snakelike course taken by a stream in which erosion is greatest on the outside or convexed portion often forming a cut-bank while deposition is greatest in the inside or concave side often forming point bars
click here to see a meander
as the outside portion of a meander continues to erode the neck of land between, the outer portions become more narrow finally causing the looped end of the meander to be isolated--this cutoff section is called a meander cutoff or oxbow
an oxbow will typically form a lake in the early stage of development and will form an oxbow (or meander) scar over a period of time in which the oxbow lake fills with sediment
click here to see steps of oxbow formation --- see page 477 in text for another example of oxbow formation
click here to see oxbow lakes
click here to see a photo of oxbow scars
a meander whose channel is deepened by downward erosion is called an entrenched (encised) meander and may be caused by subsequent rejuvenation or uplift of the land and stream channel
click here to see a photo of an entrenched meander on the Colorado River in the Grand Canyon – see page 489 in text for another example of encised meander formation

B. Alluvial fan and delta

as mentioned before as a stream widens it will decrease in velocity and deposit its load
a high velocity stream will spread out as it flows from the mountains into the valley and as a result will deposit its load in the form of a fan--the same mechanism causes a delta to form at the juncture of a large river and the ocean--examples of deltas are the Mississippi, and Amazon deltas
click here to see an alluvial fan
click here to see deltas

C. Floodplain and natural levee

a floodplain is a relatively flat region flanking most streams and formed by deposition of finer grained sediments during periodic flooding of the stream waters
a natural levee is a ridge or platform built on the floodplain adjacent to and trending along the course of the stream and represents the highest area or point on a floodplain
natural levees are caused by successive flooding episodes and can inhibit subsequent flooding
click here to see floodplain with levees – see page 483 in text for another example of levee formation
click here to see a photo of a floodplain
click here to see a photo of levees

D. Channelization

is the structural alteration made to a stream's channel in order to speed the flow of water and thus prevent it from flooding--channelization usually involves cutting off meanders to straighten a stream--the shorter straight channel will have a steeper gradient than before and its increased velocity will transport more water perhaps enough to prevent flooding in an area
click here to see channelization of a stream and synthetic meander cutoffs