Running Water

I. Introduction

A. The hydrologic cycle is the unending circulation of Earth’s water, driven largely by energy from the Sun. The cycle describes the storage and movement of water near Earth’s surface that is it describes the reservoirs in which water resides and the processes that move it around. The ocean is by far the largest reservoir, followed by glaciers and then by groundwater. Although these volumes are relatively constant, significant amounts of water move continuously in and out of these reservoirs. 

 

Reservoir

Percentage of Earth’s Water

Ocean

97.2

Glaciers

2.15

Groundwater

0.62

Freshwater Lakes

0.009

Saline lakes/Inland Seas

0.008

Soil Moisture

0.005

Atmosphere

0.001

 

B. This circulation of water from one reservoir to another is     called the hydrologic cycle.

1. Water moves out of the ocean via evaporation and is carried by atmospheric currents of air over the land masses where precipitation occurs in the form of rain and snow. 

2. Water is carried by run-off along the surface of the ground and eventually runs into rivers or seeps down into the ground to become part of the subsurface water reservoir.

3. Water flows in rivers and in underground conduits from higher to lower elevations, towards the sea.

4. Water also evaporates directly from the land surface or is returned to the atmosphere by plants in a process collectively called evapotranspiration.

C. Average annual USA water budget:

              Precipitation            76 cm

              Evapotranspiration       53 cm

              Run-off                  23 cm

              Infiltration          0.025 cm

 

D.  Rivers contain much less than 1% of the total water on Earth. Their importance stems from their major role in the transport of water, dissolved solids and suspended matter. In this light, they are much more important than the other transport agents - the atmosphere and glaciers.

E.  Rivers are the major routes by which continental rain and the products of continental erosion reach the oceans.

 

II. Stream flow - key concepts

A. The complex interactions between flowing water, channel shape, climate, channel material, etc. cannot be adequately expressed mathematically. The multitude of variables that define the system are constantly adjusting and readjusting to minor variations in flow. Scientists studying river flow have often had trouble determining what was causing observed changes. It is also often difficult to determine, which are the dependent and which are the independent variables.

B.  Turbulent flow of water in streams is by far the most important type of fluid flow observed there.  When the velocity of the stream exceeds some critical value the flow changes from laminar to turbulent.

1. In turbulent flow, as the velocity of the water increases, the individual water particles follow a swirling chaotic path with eddies superimposed on the main forward flow.

C. The velocity of the stream water is equal to the distance the water travels in a given amount of time:

                 Velocity  =  Distance / Time

     1. The typical range of stream velocities is

                        15  -  750  cm/sec

                      0.32  -  16  mph

D. Factors determining stream velocity

1. It's important to understand the factors which determine stream velocity because erosive capability of a stream is proportional to velocity.

a. The faster the stream flows, the more it can erode.

2. Many factors affect the velocity of a stream, but a generalized formula can be used to describe most of them:

 

                              G  A

                      V = √ -------

                              R  P

 

     V = velocity

     G = Gradient

     A = cross-sectional area of the channel

     R = roughness of the channel floor

     P = wetted perimeter (total length of that part of the             channel sides and bottom that are underwater)

 

a. A change in one of these factors can cause changes in the other factors, thus contributing to the complexity of stream flow.

3. Gradient or slope - the drop in vertical elevation of the stream's surface for a specific horizontal distance the stream flows.

a. Geologists used to think that this was the predominant factor determining velocity, but depth may be more important.

b. Usually measure in feet of elevation drop / miles of horizontal distance traveled.

c. It's measured just like you would measure the gradient of a hill.

d. Longitudinal profile- cross section of stream along its course from headwaters to mouth.

e. Generally a stream has a steeper gradient near its headwaters at its highest elevation giving it a concave upward profile.

f. Low gradient = 5 cm/km = 3"/mile = parts of Mississippi River

g. High gradient = 6600cm/km = 350'/mile

4. The force of gravity that drags the water downhill from the higher elevations must compete w/ frictional forces generated between the water and the stream bed. The magnitude of these frictional forces varies with the size, shape and roughness of the channel.

a. Consider the following different channel shapes, all with the same cross-sectional area (10 units2)

1) Wide, shallow channel (Perim. = 12 un)

2) Deep, narrow channel (Perim. = 12 un)

3) Semicircular channel (Perim. = 8 un)

     5. Summary of how the above factors affect velocity

a. Steeper gradient (G)               = higher velocity

b. Larger cross-sectional area (A) =    "        "

c. LOWER PERIMENTER = LESS FRICTION =   "        "

d. Less channel roughness (R)=smoother =  "      "

6. The competition between gravity and friction results in velocity differences within a single stream. The velocity is higher for water that travels in the stream where the friction is the least.

a. For example, in a straight channel, its is fastest in the center, near the surface and slowest along the bottom.

b. Also, in a curved channel the velocity is highest around the outside of the bend.

c. The maximum turbulence occurs where these two opposing forces go head to head such as along the walls and bed of the stream.  And as it turns out, the more turbulent the flow the higher the eroding capability of the stream

E. When you put all this information together you come up with the concept that the parts of the stream where the velocity is the highest and the interaction with the stream bed is the most intense- the most erosion occurs.  That is, the most intense erosion occurs on the outside of the bends of a curved stream channel or along the bottom and the edges of a straight stream channel.

F. Another important factor that affects stream velocity is DISCHARGE, which is the volume of water flowing past a fixed point (gauging station) in a specified amount of time.

1. Discharge varies from stream to stream, and from time to time AND place to place in a single stream.

a. It is usually higher in the spring due to melting of winter snow.

b. Usually higher farther downstream as more tributaries (smaller streams that flow into larger stream) merge into the big stream.

c. Usually higher during the rainy season in a region that has distinctly wetter and drier seasons of the year.

G. A diagram called a HYDROGRAPH is used to display the variations of stream discharge with time, at a specific location.

1. Hydrographs can show yearly, monthly, daily or instantaneous discharges and periods of high and low discharge can be determined from them.

2. Hydrographs compiled over long periods of time are useful for designing irrigation and power systems, and for predicting water-supply patterns and for forecasting floods.

H. Flooding

1. When the amount of water in a stream exceeds the capacity of the stream bed the water rises up over the banks and floods the adjacent lowland called floodplain

a. Bankful stage=maximum capacity of stream bed that just fills stream channel.  Observations have shown that a stream rises to its bankful stage about once every 1.5 - 2 years. The size of the stream channel is adjusted to handle the volume of flow that occurs once every 1.5-2 years.  

2. Floods are a natural, recurring phenomenon.  The time elapsed between occurrences of a particular size flood is predictable if historical data is available. In other words, floodplains flood periodically, and we must keep this in mind when we contemplate building on a floodplain.

3.  Perhaps the greatest immediate practical use of stream monitoring and hydrographs is to provide knowledge of the magnitude and probable frequency of floods.

a. A simple statistical method enables hydrologists to construct a diagram such as this that allows them to predict flood recurrences.

4. Flood-control dams help to modify the shape of the hydrograph and to lessen the impact of flooding on downstream regions by controlling the rate at which flood water is allowed to go downstream.

5. Urbanization and agriculture usually have an opposite negative effect on stream flow.

a. Effects of urbanization

1) Decreases the amount of water sinking into the ground because it is sealed off by concrete buildings, streets and parking lots.

2) Increases the amount of run-off.

3) Drops the level of groundwater.

b. Effect on stream flow

1) The time lag between precipitation and flood peak is decreased because there is no infiltration and less vegetation to slow the water down.

2) The flood peak is higher because the stream must carry more water in a shorter time.

c. Urbanization often creates flashy streams. That is streams with a low normal flow and a high, short flood peak.

I. Base Level

1. Another key concept in the study of stream flow is base level, which is the lowest point to which a stream can erode its channel.

a. The ocean is the ultimate base level and as we have seen, since sea level fluctuates, this ultimate base level does not have a permanent elevation.

2. Alternate definition-the elevation or level at which the mouth of the stream enters a large standing body of water (lake or ocean), and disappears as a river.

3. Graded stream

a. Nature looks for the easiest way to get her work done, which includes getting water from high elevations down to the ocean.  Therefore, over the course of time a stream deposits and erodes its channel to achieve the most efficient profile for carrying its sediment load. A stream that has achieved this most efficient profile is called “graded”.

4. A stream can have a number of local base levels (e.g., lakes or waterfalls) that affect streamflow directly upstream of them.  Along the course of the river upstream from these local base levels the river can not erode below the elevation of the lake or waterfall.

a. These local base levels are temporary and disappear when the lake or waterfall or dam disappears.

5. Artificially raising or lowering the base level of a stream affects the stream flow, gradient and channel characteristics.

a. If the base level is raised such as by building a dam, the upstream gradient becomes less steep, this leads to decreased stream velocity, decreased energy available to transport sediment and results in deposition of material.  This is one factor that contributes to the "filling-in" behind dams.

b. Downstream from the dam, the gradient of the stream is increased which has the opposite effect and leads to erosion of material below the dam.

 

III. The work of running water

A. Introduction, The work or funning water includes:

1. Transportation of sediment

2. Erosion of river channel

3. Deposition of sediment

B. Transportation

1. Terminology of stream carrying characteristics

a. Load-amount of material a stream carries at any time.

b. Capacity-the maximum load of particles of any size that a stream can transport.

c. Competence- a measure of the largest grain a stream can transport. It is a measure of the ability of a stream to carry particles of diff. sizes.

1) Higher competence is usually associated with a higher stream velocity.

     2. Ways in which streams carry particles

         a. Solution - the dissolved load, carried as ions

1) Commonest compounds carried in streams: Ca, Mg, HCO3, Cl, SO4, NO3, Si

2) Amount of dissolved material varies with:

                   a) Climate

                   b) Season

                   c) Geologic terrain being eroded

b. Suspension - Turbulent currents lift particles up into the flowing water.  Particles carried in this way are said to be in suspension.

1) Turbulence increases when velocity increases so the greatest amount of material of the largest size can be carried during floods.

2) Mainly mud and silt are carried in suspension.

c. Bed load - particles that roll or slide along the stream bottom make-up the bed load.

              1) Mainly sand and gravel are carried in bed load.

d. Clastic load = suspended + bed load

1) Greatly increases during periods of increased discharge.

C. Erosion

1. Streams erode material by a number of different means:

a. Streams dislodge material from their beds and carry it away.

b. Abrasion - Solid particles carried by the stream act as erosive agents by wearing down the bedrock.

1) Also the impact of large particles knocks fragments off the bed.

c. Dissolves channel debris and bedrock.

D. Deposition

1. When the velocity of the stream decreases below that needed to keep material in suspension, deposition begins.

a. Alluvium - any stream-deposited material.

2. Channel deposits - any alluvium deposited within the bounds of the channel.

a. Bar - any such deposit

b. Point Bar - bar on inside of bend

3. Braided stream results when sediment load is large with respect to discharge, gradient, and channel depth.

a. The shallow channels tend to become filled & streams break through their walls & form new channels. What forms is a complex network of converging & diverging channels that thread their way among bars

1) Many of the subsidiary channels will rejoin further downstream giving the stream a familiar pattern that gets the name braided.           

b. For example, if a steeper, more turbulently flowing tributary enters a main stream, its rocky bed load may be deposited at the junction. Excessive load may also be provided when debris from barren slopes is flushed into a channel during a heavy downpour, or at the end of a glacier

c. Braided streams also form where there is an abrupt decrease in gradient of discharge.

E. Floodplain deposits 

1. Natural levees - elevated land forms made of alluvium and deposited immediately on either side of the stream channel

a. Lower Mississippi R. = 20 ft.

b. They parallel streams and confine waters near the river creating marshy conditions (back swamp)

c. Natural levees can also prevent tributaries from making their way into the main channel, so that they are forced to flow through the back swamp zone parallel to the main river for many kilometers = Yazoo tributary

F. Delta deposits - accumulation of sediment formed where a stream enters a standing body of water (e.g., lake or ocean).

1. Relatively flat, barely above the water surface.

2.  The sedimentary structures observed in a delta reflect the different particle sizes being deposited and the differing energy levels available to the stream for depositing them.

3. As the delta grows outward, the stream's gradient continually lessens, which decreases its ability to carry sediment. Therefore, the channel becomes choked with sediment and the river seeks a shorter route to base level. 

4. Main channel also divides into smaller ones called distributaries, which do the opposite job to that of tributaries.  Instead of feeding into the main stream, they carry water away from the main channel.

5. The delta of the Mississippi River began forming millions of years ago near the town of Cairo, Illinois. Since then it has advanced nearly 1000 miles south.

a. The Mississippi River delta is actually a series of 7 coalescing subdeltas.

b. For many years the Mississippi has been trying to cut through a narrow neck of land and shift its course into that of the Atchafalaya River.  If this happens the river will abandon 500 kilometers of its path in favor of a much shorter route. Only the construction of massive dams has prevented this from happening.

G.  Alluvial fan deposits - fan-shaped deposit formed where a stream's gradient is abruptly reduced. These commonly form where a high-gradient stream leaves a narrow mountain valley and passes out onto a broad, flat plain or valley floor.

1. The sudden drop in velocity causes the stream to drop its load in a fan- or cone-shaped deposit.

2. The dryland equivalent of a delta, but can be steeply sloping

 

IV. Features of stream valleys

A. If not for mass wasting all stream valleys would have straight, vertical walls. However, mass wasting generates a characteristic V-shaped cross-section.

B. The features developed in stream valleys depend on:

1. Geologic Age of region

2. Tectonic activity

3. Bedrock composition

4. Climate - affects mass wasting & weathering

5. Because of these factors, stream valleys vary, but there are two main types.

C. Narrow, V-shaped valleys

1. Valley walls - rise on either side of floodplain to the crests of the flanking hills or mountains which are called:    

a. Divides - separations between valleys.

2. Rapids form where there is a sudden gradient increase

3. Waterfalls form where there is an extremely large & sudden increase in gradient.

a. Not all falls form in the same way.

1) Niagara Falls - resistant layer overlying less resistant layer

2) Hanging Valley Falls in glaciated regions form where a smaller tributary glacier entered a larger glacial valley

b. These "knickpoints" in the stream's profile are actually the  cause of their own disappearance, since the steep slope results in shooting flow, with increased erosion from impact, eddies,  and cavitation.

D. Wide valleys with flat floors

1. Meanders - form by a combination of erosion and deposition

a. Obstruction forces current against bank

b. Outside of bank erodes due to increased turbulence

c. Material carried downstream & deposited in center of stream & on inside of next bend

          d. They migrate across and down valley

2. Cut bank - outside of meander

3. Cut-offs - new, stream channel formed when the narrow          neck of a meander is cut through and the stream abandons          the old bend.

4. Ox-bow lakes = Water-filled abandoned meanders

5. Meander scars = Dry abandoned meanders

 

V. Drainage Networks

A. Drainage basin - entire region from which a stream and its     tributaries receive their water.

         1. Every stream, no matter how small, has one.

B. The type of drainage network that develops depends on composition and structure of the underlying rock.

1. Dendritic = tree-like = develops in areas where surface materials all erode at the same rate (i.e. where the underlying strata are horizontal and the same material is exposed for large distances.) This situation is found in regions underlain by flat-lying sedimentary rocks & massive, homogenous igneous and metamorphic rocks.

2. Radial = streams radiate out in all directions from a central high = domal uplift or volcano.

3. Rectangular = differential weathering of fractures or joints in bedrock localizes flow into an ordered geometric pattern.

4. Trellis = forms when bands of rocks resistant to weathering alternate with bands that erode more rapidly.  This is typical where rocks have been deformed into a series of parallel folds and are, therefore, very common in the Appalachian Mountains.

C. To understand fully the features of a drainage network it is often necessary to study the history of the region.

1. Water gap - Steep-walled notch followed by a river through a raised structure in the landscape

2. Why does the river go through the mountain instead of around it?

a. Antecedent stream - existed before the structure, which was raised later while the stream continued to downcut. Sequence of events:

1) Stream forms

2) Tectonic Uplift generates ridge

3) Downcutting of stream keeps pace with uplift

b. Superposed stream - stream let down upon structure, which already exists in the rocks that underlie the unit the stream is cutting into.

3. Headward erosion and stream piracy

a. Streams can lengthen their course by headward erosion extending into previously undissected land.

b. Can lead to stream piracy and wind gaps.

 

VI. Stages of Valley Development

A. Until recently geologists believed that river valleys evolved through predictable stages of development as time passed. Recently, however, this concept has fallen out of favor with fluvial geomorphologists. It is now believed that the features developed in a stream valley result from a much more complex interaction of factors and don’t just follow one another predictably as time passes. In nature there are no clearly definable differences in the features found in streams of different ages. Instead, the features developed in a stream valley are much more dependent on tectonics, climate, and bedrock, than on the number of years since they originated. For example, it may take 106 years to develop rapids and waterfalls along a stream cutting into granite, whereas, in a region of more easily erodable mudstone a valley may evolve to this point in a much shorter period of time.

B. What is observed is that a stream that is actively downcutting rather than eroding laterally, is typified by the presence of a narrow stream channel that almost completely fills the stream valley (i.e., not much of a floodplain), rapids, waterfalls, few meanders, and a steep gradient.

C. A stream that more closely approaches base level increases the amount of lateral erosion and decreases the amount of downcutting it is doing. This increases the valley width so that it may greatly exceed the channel width. Also, meanders, cut-offs, oxbows, a lower gradient, and a smoother profile develop.

D. A stream that is mainly reworking previously-deposited alluvium, which is easier to do than downcutting, tends to meander extensively over an exceptionally wide floodplain. For example, the lower Mississippi River meanders migrate at a rate of ~20m/yr. Such streams are seldom near their valley walls so the valley doesn't widen anymore. That is, there is virtually no more lateral erosion of the valley walls and the only downcutting that occurs is into previously-deposited alluvium. The stream gradient is low and oxbow lakes, meanders, cut-offs, natural levees, yazoo tributaries, and back swamps are common.

E. A stream may be rejuvenated and begin vertical downcutting     again because  of a change in base level caused by processes such as tectonic uplift or decreasing sea level, both of which increase gradient. In this situation entrenched (or incised) meanders and stream terraces can result. The Grand Canyon of the Colorado is an example of a rejuvenated stream.