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Abstract 

Concepts of stream dynamics are demonstrated through discussion of processes and process indicators; theory is included only 

where helpful to explain concepts. Present knowledge allows only 

qualitative prediction of stream behavior. However, such predictions show how management actions will affect the stream and 

its environment. 
USDA Forest Service 

General Technical Report RM-72 

Stream Dynamics: 

An Overview for Land Managers 

Burchard H. Heede, Principal Hydraulic Engineer 

Rocky Mountain Forest and Range Experiment Station1 

April 1980 

'Headquarters at Fort Collins, in cooperation with Colorado State University. Research 

reported here was conducted at the Station's Research Work Unit at Tempe, in cooperation 

with Arizona State University. 
Contents 

Introduction ..................................................................... Scope and Purpose ............................................................. Characteristics of Streams ...................................................... The Basic Fluvial Process ......................................................... Subcritical Versus Supercritical Flow ............................................ Froude Number .............................................................. Laminar Versus Turbulent Flow .......................................... ;....... Reynolds Number ............................................................ Sediment Transport ............................................................ Interdependency of Hydraulic Variables .......................................... Equilibrium Condition and Adjustment Processes .................................... Dynamic Equilibrium ......................................... :................. Complex Channel Response ...................................................... Major Adjustment Processes ...................................................... Processes Affecting Channel Pattern and Shape ................................... Bar Formations .............................................................. Channel Patterns ............................................................. Channel Shapes .............................................................. Width-Depth Ratio ......................................................... Shape Factor .............................................................. Avoiding Pitfalls of "In-Stream Flow" Investigations ........................... ChannelBanks ............................................................... Processes Affecting Longitudinal Profile .......................................... Longitudinal Profile Relationships ............................................. Aggradation ................................................................. Degradation ................................................................. Armoring .................................................................... The Need to Monitor Stream Behavior .................. ........................ .,,. Conclusions ...................................................................... Literature Cited ................................................................... Notation ......................................................................... 
Stream Dynamics: 

An Overview for Land Managers 

Burchard H. Heede 

INTRODUCTION 

Scope and Purpose 

A land manager should understand general concepts 

of stream behavior to be aware of how management 

actions affect streams, since such actions can influence 

streams drastically. For example, vegetation cover 

manipulations on a watershed can increase water yield 

(Hibbert 1971), or streamside forest removal can activate bedload transport (Heede 1977). Streams are 

dynamic systems that are prone to change even without 

human interference (Heede 1975); change in the system 

itself may bring about adjustment once a geomorphic 

threshold has been reached (Schurnrn 1974). Thus, land 

management-stream interactions are complex. 

Other professionals who need a general understanding of fluvial processes are hydrologists, civil 

engineers, fishery biologists, and plant biologists. Plant 

biologists might be concerned with riparian vegetation 

interactions with the stream, while the fishery biologist 

works in the stream itself. The hydrologist, on the other 

hand, is concerned with precipitation input into a 

watershed, its concentration in channels, and the 

mode, quantity, and quality of flow discharge. The civil 

engineer is more likely to be concerned about scour at 

bridge piers (degradation), or loss of discharge capacity by deposition (aggradation). 

Knowledge of stream dynamics will help to point out 

what information is needed to characterize stream 

conditions, and what indicators suggest about present 

and future stream behavior. With this knowledge, professionals engaged in land management will be able 

to estimate what, where, and when effects of their 

actions will take place. 

General understanding of fluvial concepts should 

also help managers recognize if a particular situation 

demands attention from an expert in river science (a 

potamologist). Unfortunately in many situations, 

stream data are either nonexistent or cover only flow 

rates and yields, but do not characterize present conditions or make well founded projections possible (Heede 

19 79). 

High- and low-flow conditions have different requirements for conveyance of the water. If flow changes 

drastically, the channel often does not meet the new 

requirements, and adjustments must be made through 

stream-erosion processes. Well known examples are 

channel bed degradation or channel meandering. 

This report presents a general outline of water flow 

and sediment transport as basic to all channel 

changes. It describes stream equilibrium condition and 

the required adjustment processes if equilibrium is 

lost. These processes lead to alignment, shape, or 

profile changes. 

The user should be acquainted with the basics of 

flow and sediment transport and should recognize the 

strong interdependency between practically all 

hydraulic variables. Arrangement of this text permits 

the reader to consider only those processes of interest. 

The Literature Cited section will suffice or provide a 

starting point for in-depth study of specific processes 

or theories. 

Characteristics of Streams 

We usually think of a stream as one entity with a 

specific characteristic, such as the mighty Mississippi, 

the unpredictable Platte River, a small artery of a large 

stream system, or a bubbling mountain brook. But these 

same streams are very different at different times. 

At high-flow stage, for instance, mountain brooks are 

anything but bubbling waters. At low-flow regime, they 

meander sluggishly between channel banks, often filling not more than a fraction of the total bed. But at 

high-flow regime or flood stage, the channel may be 

totally filled or even too small, flood plains become 

water covered, and meanders straighten to convey the 

rushing waters most quickly. 

If we would analyze the hydraulic variables of these 

two types of flow, we would have to conclude the 

character assigned to the stream is based on a single 

aspect and not on its complexity, because two different 

streams may be represented if low- and high-flow 

regimes are considered. Although the argument may 

appear merely semantic, in reality it is deep seated, 

since different processes are operative in both 

streams. Even to the casual observer, stream systems 

are dynamic. 

THE BASIC FLUVIAL PROCESS 

Subcritical Versus Supercritical Flow 

Natural streams convey their water in different 

modes. At times, or in certain channel reaches, the 

flow is tranquil (subcritical) exerting low energies 

on banks and bed; at others, shooting (supercritical) 

flows occur which energies may damage an unprotected channel. Human activities can cause undesirable high-energy (supercritical) flows. For example, 

encroachments into the channel by diversion structures 

or bank revetments can narrow the channel. As a 
consequence, the area of flow may decrease to the 

point where shooting flows occur. Knowledge of flow 

regimes, describing the flow characteristics in terms 

of available energies, therefore is required to 

recognize problems in channels or proposed channel 

improvements. 

Fortunately, subcritical flow is the more common 

type of flow (fig. 1). Available energy of such flows is 

less than that of supercritical flows of the same discharge. The total energy of a flow (total head of flow) 

can therefore be better conserved in subcritical flow 

(Koloseus 1971). Maintenance of total head is important 

for channel stability as well as for structures such as 

water diversions. 

In contrast, supercritical flow is undesirable because 

of its great erosive power due to high velocities (fig. 2). 

Also, higher stagnation pressures of supercritical 

flows can develop uplift forces of such magnitude that 

canal linings and concrete diversion structures have 

been removed (fig. 3) (Koloseus 1971). Stagnation 

pressure is the sum of the pressure intensity in a zone 

of uniform motion and the rise in pressure intensity due 

to channel obstruction or other changes. 

Supercritical flows can also cause standing waves, 

~roduced when two equal waves travel in opposing 

directions. Standing waves may reach magnitudes 

requiring prohibitively high canal walls or diversion 

ditchbanks. Standing waves must also be considered 

where narrowing of channels by human works is 

planned. As a general rule, stream reaches with supercritical (shooting) flow should be avoided for structural 

installations, and this type of flow should not be created 

by channel manipulations (narrowing). 

Froude Number 

where V is the average velocity in the cross section of 

measurement, g if the acceleration due to gravity, and 

d is the average water depth. If inertia is smaller than 

the gravitational force, Fr<l, and flow will be subcritical (fig. I), the flow will be supercritical if the ratio 

reverses and Fr>l. Critical flow, required for many 

artificial stream-gaging stations, has a Froude number 

of 1, but seldom will occur in natural channels. This 

regime is very unstable and generally occurs only for 

very short periods of time as a transitional stage between tranquil and shooting flows. While subcritical 

flows will show a relatively smooth water surface 

except for waves created by winds or some protrusions, supercritical flows exhibit surface waves that 

travel upstream (fig. 2). 

Laminar Versus Turbulent Flow 

Flows can also be characterized by the movements of 

individual fluid elements with respect to each other, 

which result in either laminar or turbulent flow. The 

major distinction is that, in turbulent flows, a complex 

secondary motion is superimposed on the primary 

(laminar) motion of the fluid elements (Rouse 1950). 

In laminar flow, each fluid element moves in a 

straight line with uniform velocity. There is no diffusion between the layers or elements of flow, and 

thus no turbulence. In contrast, turbulent flow has a 

complicated pattern of eddies, producing random 

velocity fluctuations in all directions. These are caused 

by continuous interchange of finite masses of fluid 

between neighboring zones of flow. The phenomenon 

results in a disruption of the entire flow pattern; a 

current meter immersed in a turbulent stream shows 

continual deviations from the mean value. Obviously, 

constant changes of flow lines lead to surges of flow 

against banks and structures, increasing flow impacts. 

Turbulent flow is the normal condition in streams. 

The Froude number (Fr), a dimensionless parameter, 

offers a quantitative measure to determine if subcritical or supercritical flow will occur. This number 

represents the ratio of inertial to the gravitational 

forces and is given by: 

Reynolds Number 

Movements of the fluid elements depend on the 

inertial and viscous forces. The latter, in a loose sense, 

could be envisioned as an expression of internal friction 

of the flow. The Reynolds number (Re) is a dimensionless 

measure of these forces: 

Vd inertial force Re=-= 

v viscous force 

where v is the kinematic viscosity represented by the 

ratio pl~; ,u is the absolute viscosity and Q the fluid density. For natural channels, the critical value dividing 

laminar from turbulent flow is near a Reynolds number 

Figure 1.-Downstream view of the Verde River north of Phoenix, of 2,000; values less than 2,000 generally indicate 

Ariz. The flow is subcritical (tranquil). laminar and those over 2,000 indicate turbulent flow. 
Figure 2.-Shooting flow in a constricted segment of the Salmon River, Idaho. The view is 

downstream. Note waves are traveling upstream (toward the viewer). 

In summary, most natural streams have subcritical 

flow (Fr =<I). Also, turbulent flow (Re = >2,000) prevails over laminar flow (Re = <2,000). 

Sediment Transport 

Sediment transport is very complex. At least 30 

variables are locked into the sedimentation processes, 

and the degree of interdependency between these 

variables is not fully understood. It is not surprising, 

therefore, that a reliable numerical method for determining bedload transport in alluvial streams is not yet 

available (Bogardi 1974, p. 17). Available methods are 

Figure 3.-High stagnation pressures from shooting flows of the 

unusual January 1979 flood in the Verde River, central Arizona, 

created uplift forces that aided in upsetting this water-diversion 

structure and a 2-foot-diameter concrete pipeline. 

based on empirical relationships between a selected 

number of variables or require prerequisites based on 

assumptions not necessarily valid for the conditions. 

For stable channels, Lane (1955a) demonstrated the 

qualitative relationships between sediment discharge 

(Q,) and water discharge (Q,) as a balance (fig. 4) 

where D is the sediment particle size and S represents 

the slope. Lane's balance illustrates the intricate 

stream adjustment processes. If one side of the balance 

changes, the other must adjust to maintain equilibrium. 

Thus alterations in sediment or water discharge require changes in grain size andlor slope. For example, 

if the slope is increased, the stream will attempt to 

transport larger particles, or if stream discharge 

decreases, sediment load must decrease also to maintain balance. Otherwise, degradation or aggradation of 

the streambed will occur, respectively, as indicated by 

the figure. 

For his balance, Lane selected as sediment discharge (Q,) the coarser part of the sediment load, or 

more exactly the bed material load. He defined bed 

material load as the sediment in transport of sizes 

readily available in considerable quantities in the 

streambed. It is the coarser material largely molding 

the bed formation. He argued, "in most cases, the 

quantity of the fine load of silt and clay sizes can 

change almost indefinitely without materially affecting 

the river profile." We know now there are exceptions 

to this rule, such as when extremely large loads of fine 

material (wash load) are available. Heavy concentrations of fines cause a higher apparent viscosity of the 

fluid, which in turn results in increased transport 
Figure 4.-Stable channel balance. (Sediment loads times sediment size) varies as (stream 

slope times stream discharge). (Courtesy of the American Society of Civil Engineers; from 

Lane, E. W. 1955. The importance of fluvial morphology in hydraulic engineering. American 

Society of Civil Engineers Proceedings, Hydraulic Division 81:745-1 to 745-17.) 

capacity for sediment. More and larger particles can 

be transported, which in turn affect the stream profile. 

An extreme condition, illustrating the viscosity influence (Bull 1968) in addition to the relatively high 

specific gravity of the flow mixture (Crandell 1968), 

is a mudflow carrying room-size boulders like soap 

bubbles. Johnson (1970, p. 461) suggested that lift 

forces may also be involved. 

Flume experiments by Simons et al. (1963) demonstrated that clay in suspension affects viscosity, bed 

form, and flow resistance. With clay suspension, the 

total sediment load increased, which in turn affected 

the bed form. Since bed forms influence flow resistance 

(Simons aria Richardson 1966), they enter directly into 

the hydraulics of the flow. Again, the interdependency 

of the processes illustrates the difficulty, if not impossibility, of quantitatively predicting the stream 

environment. 

Water temperature also influences sediment 

transport capacity more than many investigators 

recognize. As temperature decreases, viscosity increases, and carrying capacity increases [Colby 1964). 

For example, when Colby dropped water temperature 

from 80° to 40° F, the sediment discharge for sand 

sizes ranging fram 0.125 to 0.250 mm increased by 

254%. The size class is common for riverbed sand. 

Field investigations confirmed the temperaturesediment transport relations and their vital influence 

on stream behavior. In the Lower Colorado River for a 

given discharge, the sediment loads were 2.5 times 

greater in winter than in summer (Lane et al. 1949). 

Fahnestock and Maddock (1964) reported a change in 

water temperature of about 35O F from March to 

August significantly affected the hydraulics of flow 

and bed form in the Rio Grande River near El Paso, Tex. 

At some similar discharges. mean velocity was greater 

in March and the bed was plane, whereas mean velocity was less in August and large dunes developed on the 

bed (flow resistance increased with dune formation). 

Management implications based on the temperaturesediment relations are many. Where data on annual 

sediment loads are required, best results will be 
obtained by sampling during summer and winter. 

Winter data of one stream should not be related to 

summer data of another. Most land management actions result in increased water temperatures and, 

therefore, decreased sediment loads. Under extreme 

conditions, this could lead to aggradation. 

From many observations it has been found the sediment load in natural streams varies roughly as the 

square of the discharge (Lane et al. 1949). This approximation is expressed by the equation 

where Q, is the sediment load in tons per day, Q, 

represents the water discharge in cubic feet per 

second, and K, is a coefficient that normally changes 

from stream to stream and with the season. The K, 

coefficient could be helpful for approximating future 

sediment loads if used with flow frequency curves (differentiated between winter and summer flows), 

because some probabilistic aspects of sedimentation 

would be taken care of. Because the variation in the 

original data will generally be large, long periods of 

record are required to obtain meaningful averages. 

The length of record required depends on the degree of 

uniformity in the particular stream system. 

Differences between sediment transport capacity 

and actual sediment load lead to deposition or scour 

(Lane 1955b). In other words, sediment is stored or 

depleted in a given reach, resulting in nonuniform flow 

of sediment. Uniform sediment flow represents equilibrium condition and satisfies the continuity principle of 

sediment (Vanoni et al. 1961). If actual sediment 

discharge differs from the equilibrium sediment 

discharge, the stream will try to adjust the actual to 

the equilibrium discharge. Both the actual rate of 

transport and the equilibrium rate will change, but 

they will tend to approach each other. Uniformity of 

sediment discharge must be considered over time, 

because sediment often moves in a wave-like mode, 

especially if movable bed forms such as dunes are 

present. 

In many instances, works of humans distort uniform 

sediment flow. Best known examples are river dams 

causing a decrease of equilibrium transport rate 

upstream from the reservoir (backwater zone), and a 

decrease of actual transport rate below the dam. 

While aggradation takes place in the backwater zone, 

the "starved" stream picks up material from the bed 

downstream from the dam to satisfy the equilibrium 

requirement. 

Vanoni et al. (1961) roughly categorized river 

engineering problems caused by nonuniform sediment 

transport. When the actual transport rate is larger 

than the equilibrium rate (deposition), the following 

occur: 

1. Aggradation upstream from a reservoir. 

2. Sedimentation in reservoirs and lakes. 

3. Tributary channels bring heavy sediment loads to 

the main channel, causing local aggradation (examples are fan formation and tributary bar) (fig. 5). 

4. Canyon streams discharge on alluvial fans, 

causing widespread deposition (as in Los Angeles 

area). 

5. Desiltinn works at water intakes that return all 

sedimentload to the main channel, in which flow of 

water is depleted, lead to aggradation (e.g., ImFigure 5.-A tributary contributed heavy sediment loads to Sycamore Creek, northeast of 

Phoenix, Ariz. These loads could not be carried by the main stream, and an alluvial fan 

developed, pushing the flow into the opposite bank. Sycamore Creek flows from right to left. 
perial Dam). Remaining flow magnitudes provided 

a small equilibrium transport rate compared with 

the original flow. 

6. Where river regulation eliminates floods that 

formerly cleared the channel of accumulated sediment and vegetation periodically, aggradation 

results in the clogged channel (e.g., Colorado River 

at Needles). 

When the actual transport rate is smaller than the 

equilibrium rate (scour), the following occur: 

1. Degradation downstream from dams that trap sediment and thus decrease the actual transport 

rate. 

2. Canals receiving clear water will scour if the bed 

material is fine enough that it can be picked up 

readily by the flow. Under such conditions, the 

equilibrium transport rate is large, while the actual transport rate of the flow entering the canal is 

essentially zero. 

3. Channel realignments that increase the slope also 

increase the equilibrium transport rate because of 

increased flow velocities. 

parameters in the equations. Attempts to reduce the 

complex stream situation by empirical relationships 

between a few variables are still pursued [Osterkamp 

1977). Obviously, such relationships will be valid only 

for the area of their origin. 

The complexity of the fluvial system suggests stream 

analysis is a probabilistic problem. Because of the 

interaction between channel bed, banks, and flow, the 

core of the problem appears to be the random distribution of the flow energy (mVZ/2), where rn is the mass 

of water. The velocity must be recognized as a vector 

force (i.e., it has direction). Because of turbulent 

flow in most natural streams, this direction occurs randomly. Thus the energy, itself of stochastic nature, acts 

on randomly distributed sediment particles. Incipient 

particle motion is therefore a very complicated event 

and no satisfactory way of determining it has been 

found. Sediment transport is a fundamental variable in 

stream dynamics. 

EQUILIBRIUM CONDITION AND ADJUSTMENT 

PROCESSES 

Dynamic Equilibrium 

Interdependency of Hydraulic Variables 

The complexity of hydraulic variables is immense. 

Lane (1957) singled out eight variables of this complex 

which he called most important: (1) stream discharge, 

(2) longitudinal slope, (3) sediment load, (4) resistance 

of banks and bed to movement of flowing water, (5) vegetation, (6) temperature, (7) geology, and (8) works of 

humans. The interrelationships among longitudinal 

profile, sediment load, and resistance of the banks and 

bed to movement are particularly close and complex. 

Interdependency between many variables often precludes the establishment of one-value relationships. 

For example, Brooks (1958) found neither the velocity 

nor the sediment discharge concentration could be 

expressed as a single-valued function of the bed shear 

stress, or any combination or" depth and slope, or 

hydraulic radius and slope. He showed experimentally 

that flow with a given depth and slope can occur with 

at least two velocities. This is due to changeable bed 

configuration which causes large variations in the 

channel roughness. Thus if one or a combination of the 

interdependent factors change under the impact of 

external or internal forces, adjustment of one, some, or 

all of the components will follow. 

It is important to recognize that established relationships between most interdependent variables, except 

for basic factors such as the shear, are empirical and 

thus not necessarily applicable for a wide range of conditions. Examples are Blench's (1966) equations based 

on the concept of the regime theory, originally developed by Lindley (1919) and Lacey (1932) for Indian 

canals. This theory assumes there exists only one type 

of cross section for a given stream with a given load. 

Thus width and depth of the channel are the main 

The concept of dynamic equilibrium is very useful in 

evaluating stream systems and their stage of development. This concept does not imply absolute equilibrium 

conditions, but that the stream can adjust to a new 

hydraulic situation within a relatively short time, 

perhaps within a few years. Obviously, if considered in 

geologic time spans, dynamic equilibrium has no place, 

because land denudation is the long-term process. 

Although dynamic equilibrium cannot be well defined, 

the investigator can use indicators, some of which are 

readily recognizable in the channel, to analyze stream 

stability in terms of equilibrium condition. 

Heede (1975) listed several factors indicative of 

small mountain streams not in dynamic equilibrium: 

channel headcuts, under-developed drainage nets such 

as those having channelized water courses only on onehalf or less of the watershed area, frequent bedscarps, 

and the absence of a concave longitudinal profile 

where watershed conditions are relatively constant. 

The last condition will depend mainly on geologic 

homogeneity, availability of bedrock outcrop, and 

tributaries. When surveying a channel, select a reach 

with sufficient length to avoid distortion of the overall 

shape of the longitudinal profile by local irregularities. 

This length will vary with size of stream. In small mountain streams, lengths of at least 0.25 mile should be 

sampled in the headwater, middle, and lower reaches. 

Obviously, greater lengths will improve the representativeness of the profile. 

Channel headcuts are local erosion sources because 

headcuts advance upstream. They indicate that stream 

length and gradients have not been developed to allow 

equilibrium condition. Bedscarps that develop at channel nickpoints similarly indicate pronounced breaks 
in longitudinal gradients. These scarps proceed upstream until a smooth transition between upstream and 

downstream gradient is attained. Thus, headcuts and 

bedscarps substantially add lo the sediment load 

because erosion will be severe until natural channelization is achieved and the longitudinal profile 

"smoothened." Actual sediment production rates 

higher than the equilibrium transport rate require 

steepening of the slope, normally attained by bed 

aggradation. Since aggradation processes are slow, 

such a stream must be judged out of dynamic equilibrium because of the long time required for reestablishment of this condition. 

Streams in dynamic equilibrium have no headcuts. 

Their watercourse begins high up on the watershed 

with a smooth transition from the unchannelized area 

to the channel. Bedscarps, not developed at locations 

of rock outcrop or variations in bedrock, are absent or 

few. As a result, sediment production is negligible. The 

longitudinal profile is concave. Generally, flow, depth, 

width, and velocity increase downstream while gradient and sediment particle size decrease if watershed 

conditions are relatively constant over long stream 

reaches. 

Complex Channel Response 

Streams respond to an upset of a given equilibrium 

condition by different adjustment processes. These 

processes follow a sequence if considered in terms 

of relative time and energy expenditures. Channel adjustments, ordered from small to large energy requirements, involve changes in: bed form, bed armor, width, 

pattern (alignment), and longitudinal profile. 

Generally, bed form changes require the least 

amount of energy and time. Examples of bed forms are 

dunes in sand bed or gravel bars in boulder-strewn 

streams. Bed forms determine resistance to flow (bed 

roughness) (Simons and Richardson 1966). Thus their 

change (i.e., from a dune to an anti-dune) may be sufficient to achieve adjustment. A dune is a sand wave of 

approximately triangular cross section in the direction 

of flow. It has a gentle upstream slope and a steep 

downstream slope. The dune travels downstream by 

the upward movement of the sediment on the upstream 

slope and the deposition of it on the downstream slope. 

An antidune is a sand wave, indicated by a regular 

undulating water-surface wave. Usually, antidunes 

and accompanying surface waves occur in trains of 3 

to 20 or more. If the antidunes move at all, it is upstream. The surface waves become gradually steeper 

on their upstream sides until they break like a surf and 

disappear. Often they reform after disappearing. 

Where a change in form roughness is not sufficient 

or is impossible, additional or different processes will 

be required to attain a new equilibrium condition. An 

additional process may be armor plating of the bed. It 

will require more time than bed form change. 

If armoring is not possible because of grain size 

distribution or insufficient amounts of large material, 

channel width changes generally would be the next adjustment process. Width increases signify increases in 

the wetted perimeter of the flow, leading to larger 

roughness of flow and smaller flow velocities. As a consequence, available energies are decreased. 

Even width changes may not bring about the required adjustment. The next most energy- and timeconsuming process would be channel pattern change 

to meander or braid.ing. If topographic and hydraulic 

conditions do not allow this change, as will be discussed 

in a later section, bed profile alterations will begin. 

Generally, these demand the greatest energy expenditures as well as the longest time of all adjustment 

processes. 

Geology, soils, and vegetation also enter into the 

selective adjustment processes because of their influence on bank and bed stability and hence on sediment 

transport. Thus they add to the complexity of stream 

response. One must realize, therefore, an orderly succession of processes, as outlined, may not operate in 

streams seeking new equilibrium conditions; the most 

energy-intensive process could be operating by itself. 

In short, the purpose of the above discussion is to show 

relative energy and time expenditures between different adjustment responses. 

MAJOR ADJUSTMENT PROCESSES 

Management activities may trigger or change channel adjustment processes. It is important, therefore, 

to recognize the adjustment criteria and factors which 

must be addressed when evaluating management activities in terms of impact on streams. 

Five major channel adjustments were outlined in 

terms of required energy expenditures in the preceding 

section. The processes responsible for those adjustments are sometimes grouped into two categories: one 

affecting channel pattern and shape, the other affecting the longitudinal profile. Channel pattern refers 

to the plan view of a stream and its alignment, while 

channel shape relates to the cross-sectional view 

across the channel. 

Some processes are reponsible for more than one 

type of adjustment, however. Thus, bar formation processes may bring about three types of adjustment: in 

pattern, shape, and longitudinal profile. In a straight 

reach, for example, alternate bars force the thalweg 

(line of maximum depth) into a meander pattern. This, 

in turn, leads to increased stream length and longitudinal profile changes, as well as to changes of the channel 

cross section. It is also obvious that aggradation or 

degradation processes change not only the stream's 

longitudinal profile but also its cross sections. The division of the processes into two categories is therefore a 

shortcoming of most of our classification systems, but it 

is accepted for convenience. The relationship between 

one process and multiple forms demonstrates again the 

complexity of stream systems. 
Four processes affecting channel pattern and shape 

are recognized. They lead lo bar formation, channel 

patterns, cross-sectional channel shapes, and types of 

banks. 

Different types of bars develop in natural streams. 

All are integrated into the stream hydraulics and are 

therefore not necessarily stationary, since most 

streams change their flows with season and individual 

events. Each type occurs in a given situation, and 

hence is an indicator of prevailing flow and channel 

conditions, as will be discussed in the following section. 

Channel patterns are very diversified, as are most 

products of nature. Three dominant patterns will be 

described-straight, meandering, and braided-and, 

as much as known, their causative processes. Straight 

channels generally offer least problems for land management, but unfortunately, they are the exception. 

The rule is the meandering channel. Channel alignment 

changes offer relatively quick and drastic adjustments 

to a new situation and are easily recognizable indicators of past, present, and possibly future channel 

developments. 

Generally, channel shapes respond quickly to 

changes in the fluvial system because shape influences 

water as well as sediment transport. Thus, channels 

closest to a semicircular shape are most efficient 

for conveyance of water and fine sediment. Coarse 

sediment requires relatively wide, shallow channels 

(Leopold and Maddock 1953). Unstable channels may 

therefore be recognized by channel shape changes. 

Parameters have been developed which permit quantitative determination of these changes. 

Consistency of bank material and availability of 

bank vegetation exert a strong influence on channel 

shape. Besides these factors, water movement within 

or over banks may play an important role in the attainment of bank stability and shape. But bank characteristics also influence channel patterns. For example, 

banks of cohesive material (clay and silt, mainly) usually 

preclude braided stream patterns, but noncohesive 

materials favor them. Bank characteristics may 

change due to lateral stream movements. Recognition 

of these changes is important because they may indicate future bank stability conditions and channel 

shape. Banks are therefore considered separately from 

shape. 

Older geologic literature overemphasized the importance of the profile in channel adjustment. As shown by 

the preceding section and stated by Wolman (19551, 

slope is not necessarily the primary mechanism by 

which equilibrium is maintained. 

Three major processes lead directly to adjustment of 

the longitudinal profile: (1) aggradation, (2) degradation, and (3) armoring. These processes act on the 

topography and the material in bed and banks. General 

relationships between profile, topography, and geology 

can be utilized to human advantage. An example is the 

use of a bedrock outcrop for gradient stabilization 

instead of a check dam. 

Generally, aggradation and degradation processes 

are undesirable, but necessary, adjustments where 

there are drastic differences between the sedimentcarrying capacity of a flow and the sediment load 

coming into a channel reach. Aggradation may lead to 

channel widening and flooding; degradation to bank 

caving and lowering of the water table in areas close to 

the channel. Armoring can stop the degradation process if sufficiently large bed material is available. In 

this sense, armoring is a beneficial byproduct of 

degradation. Amount and depth of the armor material 

below channel bottom determine the depth of degradation before an effective armor is established. 

Processes Affecting Channel Pattern and Shape 

Bar Formations 

Bars are defined as bed forms having lengths of the 

same order as the channel width or greater, and 

heights comparable to the mean depth of the generating 

flow (American Society of Civil Engineers 1966). Bars 

are deposits that may be visible or submerged, and of 

grain sizes ranging from clay to boulders. Most are 

built predominantly from sand or gravel or both. In 

longitudinal section, bars are approximately triangular. 

The upstream slopes are very long and gentle, while 

the downstream slopes are short and approximately 

the angle of repose of the bed material. Since bars are 

an active part of the geometry of flow, they change in 

size, height, or location with flow conditions. 

Generally, bar surfaces rise and fall with the 

magnitude of flow. Bars generated by high flows frequently appear as small islands during low flows. On 

sandbars, portions of the upstream slopes are often 

covered with ripples or dunes. 

Bars may be classified as point, alternating, transverse, middle, or tributary bars. The most obvious one 

may be the point bar, which develops near the convex 

(inside) bank of channel bends (fig. 6). Its shape may 

vary with changing flow conditions, but it does not 

move relative to the bend. The processes leading to this 

bar formation demonstrate the strong relationship 

between flow and bed forms. Rzhanitsyn (1960) describing the processes in bends, stresses not only the 

longitudinal currents but also the cross currents that 

influence the channel-forming processes. The actions 

of both lead toward the development of a deep pool at 

the concave (outside) bank and a bar at the convex 

bank (fig. 7). 

Two causes were thought to be responsible for the 

transverse circulation: the centrifugal force acting on 

the particles passing the curve and the difference in 

the longitudinal component resulting in a general 

spiral-like motion. The secondary circulation, 

sometimes also called helical, helicoidal, or spiral flow 

or current, continually sweeps sediment from the pool 

depression toward the convex bank, bringing clearer 

water from the upper layers to the bottom and thus increasing the erosional activity. Most of the material 

eroded from the pool is deposited on the point bar. 

These processes lead to asymmetrical cross sections in 
bends. Upstream and downstream ends of the bar are 

pointed. Intensity of erosion increases with increasing 

secondary circulation. Since this circulation increases 

with decreasing radius of curvature, the depth of the 

pool is inversely related to the radius of curvature of 

the bend. Therefore, gentle bends will have shallower 

pools than sharp bends (fig. 7). 

The maximum depth at the concave bank is greater 

than that of the straight reach. With the deepening of 

the channel at the concave bank and the reduction of 

depth at the convex bank, the principal flow gradually 

moves closer to the concave bank. There the longiludinal velocity of flow increases due to greater depth. In 

contrast, the longitudinal velocity at the convex bank is 

reduced with decrease of depth. 

High flows tend to shorten the thalweg (i.e., the flow 

selects the shortest route as it cuts across the point 

bar). This process leads to erosion at that part of the 

point bar located toward the center of the channel. The 

bar becomes narrower (fig. 6) until other flows may 

build it up again. Point bars can also be fully submerged resulting in loss of bar height, or a high-flow 

channel may be cut through the point bar (fig. 8). 

Although helical flow apparently plays an important 

role in the process of erosion and deposition in a bend, 

the mechanics of bend formation are not known. 

Meanders are known to form on the surface of glaciers 

where they are cut by meltwater carrying no sediment 

load, and point bars do not form. Meanders are also 

known to exist in ocean currents such as the Gulf 

Stream where boundaries and loads are absent. 

Leopold and Wolman (1960) argued, therefore, that the 

shapes of curves in streams are primarily determined 

by the dynamics of flow rather than by the sediment 

/ /' , Point Bar 

Figure 7.-Schematic presentation of channel shapes in gentle 

(A-A') and sharp bends B-B'). The line in the channel of the plan 

view represents the thalweg. Note in the section view the increase of pool depth with decreasing radius of curvature and 

the higher water surface elevation at the outside as compared 

with the inside bank. This is due to the gravitational force and is 

called superelevation. 

they carry. These authors compiled evidence that when 

other conditions, as yet unknown, cause a stream of 

any size flowing in a deformable channel to develop a 

meander pattern, the radius of curvature will be between two and three times the mean channel width. 

In contrast to point bars, alternating bars are not 

stationary. While point bars form in channel bends, 

Figure 6.-Point bar in Salt River north of Phoenix, Ariz. Flow is from left to right in the fore. 

ground. Note the pointed downstream bar end. 

9 
Figure 8.-In Black Canyon Creek, northern Arizona, high flows cut a channel behind this 

point bar to speed up water conveyance through this stream reach. Flow is from left to right. 

alternating bars develop in straight channel reaches, 

and are the reason why natural streams rarely remain 

straight. They "tend to be distributed periodically 

along a channel, with alternate bars near opposite 

channel banks. Their lateral extent is significantly less 

than the channel width. Alternating bars move slowly 

downstream" (American Society of Civil Engineers 

1966). The thalweg meanders between these bars, 

creating an undulating profile by forming pools and 

riffles (figs. 9 and 10). 

Alternating bars may not be visible because they are 

often partially submerged. Pools and riffles are spaced 

more or less regularly at a repeating distance equal to 

five to seven widths of the channel (Leopold et al. 

1964). Because the spacing is similar to that of bends in 

Figure 9.-(a) Schematic plan view of alternating bars (A) and thalweg (B) in a straight channel reach, and section view of the 

associated longitudinal profile (b) showing pool (C) and riffle 

formations (D). Riffles form at crossings of the thalweg between bends. 

meandering streams, these authors suggest the pooland-riffle mechanism in straight channels is the same 

as that creating meandering channels, possibly 

associated with some wave phenomenon. Normally, the 

bed material on the riffles tends to be somewhat larger 

than in the pools, but at high flows, the reverse may 

exist. 

Alternating pools and riffles were not found in high 

mountain streams of the Sangre de Christos, New 

Mexico (Miller 1958), nor the Rocky Mountains, Colorado, or the White Mountains, Arizona (Heede 1975). 

Miller argued the coarse material, left by Pleistocene 

glaciers or deposited by steep cliffs, etc., is too large to 

be effectively transported by the present-day flow regime. In Heede's streams, gravel bars were not spaced 

at a repeating distance, but distance decreased with 

increasing channel gradient (fig. 11). 

Heede's gravel bars could adjust their height with 

flow while remaining stationary; transverse bars of 

rivers move slowly downstream and occur both as isolated and as periodic forms along a channel. Generally, 

they occupy nearly full channel width (fig. 12). Transverse bars result from slope adjustment processes. 

Their main effect on the flow is increased bed roughness, as shown by strong relationships between bed 

form and resistance to flow (Simons and Richardson 

1966). 

Where the width-depth ratio is very large and the 

stream divides itself into two or several branches to 

adjust for lost depth, middle bars will be created. At 

times, they appear as submerged islands. Some are 

sufficiently long to bear willows (fig. 13). Middle bars 

are a typical bed form for braiding streams, characterized by extremely wide and shallow channels and flows 
passing through a number of small, interlaced channels 

separated by bars (Friedkin 1945). 

Tributary bars often form immediately downstream 

from tributary junctions (fig. 14). They indicate large 

sediment discharge from the tributary stream. At 

times, this discharge may outweigh the capability of 

the main stream to transport the additional load. A 

sediment fan may be built at the tributary mouth into 

the main stream (fig. 5). During a subsequent large flow 

of the main-stem river, the sediment fan may be fully or 

partially removed, and only a tributary bar may remain. 

Bars are an integral part of the hydraulic geometry 

of a stream. With the diminution or exhaustion of flood 

plains and river terraces by gravel quarrying, the 

streams themselves become increasingly attractive 

gravel sources (fig. 15). If bars are destroyed by 

natural adjustment processes, they will not form again 

unless the hydraulic geometry changes back to the 

original condition. In contrast, bars obliterated by man 

will be reestablished if a sufficient sediment supply is 

available. If not, or if reestablishment is very slow, 

adjustments such as degradation may take place. 

Degradation is generally detrimental to the environFigure 11.-Looking upstream on a transverse gravel bar (between 

arrows) in a small mountain stream (North Fork of Thomas 

Creek, Arizona White Mountains). Note the steep slope gradient 

(average 17%) that required adjustment of slope. Consequently, 

several transverse bars developed above the indicated bar. 

ment. Where transverse bars (bed forms created for 

slope adjustment) are removed, bedscarps will soon 

signify degradation is taking place. Removal of sediment will invariably destroy the bed armor, and a new 

armor can only be established by lowering the bed, 

which often leads to bank instability. 

Gravel operations may not be harmful where large 

sediment loads are available to replace bars quickly, 

thus avoiding drastic upsets of the stream system. The 

Eel River in California, one of the largest known sediment carriers in the United States, may be such a 

stream. But even in this case, we would not know the 

long-term environmental effects (redwood trees have 

lived for more than a thousand years on the periodically aggrading Eel River flood plain, as shown by 

multilayered root systems (Stone et al. 1972)). 

Channel Patterns 

Figure 10.-Alternating bars (arrows) in Black Canyon Creek. High 

flows cut small channels through the bars. Flow is toward back. In light of the interdependency of many hydraulic 

ground. variables, it is not surprising to find a large variety of 
Figure 12.-Downstream view of a transverse bar (between arrows) in the Verde River, Arizona. 

The bar is not fully exposed across the channel. Darkness of the water surface in the two bar 

openings (A and B) suggests submergence of the bar at these locations. Sediment accumulations above this bar, a natural slope adjustment structure, are indicated by shallow 

flow depth in the foreground. 

Figure 13.-Middle bars (arrows) in the Salt River near Roosevelt Reservoir bear vegetation 

such as willow and saltcedar. Flow is from left to right. At high water stages, the bars are 

submerged, while at lower stages, shown in figure, stream braiding develops. 
Figure 14.-Where the Salt River (a) is joined by tributary Verde River (b), a tributary bar (c) has 

formed. Size of tributary bars changes readily if sediment loads delivered by the tributary 

fluctuate strongly. The submerged outline of the bar at (c) suggests exposed bar size was 

reduced considerably by recession flows after the heavy load-carrying floods of 1978 and 

January 1979. 

Figure 15.-Many rivers have a substantial supply of aggregates, demonstrated by a large middle bar in the Verde River, central Arizona, that may be sought by industry. In many situations, however, removal of such material would detrimentally affect stream equilibrium. 
channel patterns because pattern changes are one 

possibility for adjustment of the hydraulic geometry to 

a new situation. This is especially true if the slope must 

change. The large variety of patterns can be reduced to 

three basic types: straight, meandering, and braiding. 

There are, of course, intermediate forms. 

Long reaches of straight channels are hard to find in 

nature. Even in straight channels, the thalweg wanders 

back and forth between the banks (fig. lo), and only the 

high flow may fill the straight reach. One could thus 

conclude that meandering and braiding channels offer 

better and easier ways for attaining equilibrium conditions than straight channels. Meandering and braiding 

increase the length of the thalweg and thus decrease 

slope. A measure of meander intensity is sinuosity, expressed as the coefficient of thalweg length divided by 

valley length. Large coefficients, therefore, express 

strong sinuosity (meandering). A straight reach would 

have a coefficient of 1. 

In braided streams, the flow is relatively shallow 

and the width-depth ratio large, larger than in meandering streams. The stream flows around many islands 

(fig. 16). If this process increases, branching of the 

main channel may occur, the flow running in different 

channels more or less parallel to each other, joining 

and anabranching (diverging) in succession. Brice 

(1975) called these "anabranching streams." 

Why channel patterns change cannot be precisely 

answered yet. Certainly, localized channel width increases may lead to meandering because of introduced 

flow deflections. A large boulder or other obstruction, 

for example, may cause a local channel width increase 

that results in a meander. But on glaciers meanders 

also appear without visible flow deflections. 

Profile is one of the major causes for pattern 

changes. Braided streams are steeper than meandering 

streams. Steeper streams have higher velocities, attack 

banks more strongly, and carry larger sediment loads. 

For bank erosion to occur, erodible bank material 

must be available. For example, braiding does not 

usually take place where banks are densely vegetated, 

but may where this cover is sparse. Although cause 

and effect are suggestive, they are not proven (Leopold 

et a]. 1964). 

In alluvial streams, whose bed and banks are formed 

by sediment transported by them, amount and character of the particles composing the sediment load also 

seem to have an important effect on channel pattern. It 

appears available load is one of the factors separating 

meandering from braided streams. Lane (1957) concluded not only overloading by sediment but also steep 

slopes are prerequisite for braiding. Overloading leads 

to aggradation, causing wide channels where banks 

are soft and, thus, the development of bars and islands. 

The width-depth ratio is larger than in meandering 

streams. But Leopold et al. (1964) pointed out that 

braiding is not necessarily associated with aggradation, since it can represent the equilibrium pattern 

(sediment inflow equals sediment outflow). Yet it can 

be argued that for braiding to occur depositions (aggradation) are required to initiate drastic channel width 

increases; with braiding, a new equilibrium condition 

may be established. 

In flume experiments, Shen and Vedula [1969) showed 

depth of flow may become so shallow due to sediment 

overloading that flow could not occur in a single channel. It had to divide into several narrow channels to 

increase depth of flow and sediment-carrying capacity. 

Figure 16.-Looking downstream on Delta River, a braided glacial stream draining the north 

face of the Alaska Range in central Alaska. Glacial streams tend to develop very pronounced 

braided patterns. (Courtesy Troy L. ~6~6, Arizona State University) 

14 
Figure 17.-Heavy sediment loads in this Salt River reach above Roosevelt Reservoir, Arizona, 

created a wide bed with shallow flow depth. Thus, stream competence (sediment-carrying 

capacity) was drastically reduced. To increase flow depth (and competence), the flow split 

into several channels (arrows). Flow in the foreground is from left to right. 

That this process takes place also in rivers was 

demonstrated during a recent recession stage of the 

Salt River, Arizona (fig. 17). 

In a glacial stream on Mount Rainier, Washington, 

Fahnestock (1963) observed braiding at much greater 

magnitude when an abundant bedload was introduced 

than when the material was derived just from bank 

erosion. Fluctuating flow discharges also favored 

braiding. 

In some situations such as mountain streams flowing 

in V-shaped valley bottoms, lateral stream movements 

are not possible. A straight channel reach may therefore remain straight. Slope adjustment is possible only 

within the given channel. In small streams, this adjustment may be achieved by the formation of transverse 

gravel bars (fig. 11) and incorporation of fallen trees 

and logs (Heede 1975). Apparently, slope is seldom adjusted by severe degradation because of the presence 

of large bed material sizes. 

The limitations set by the topography, such as 

V-shaped valleys, help to explain why braids tend to 

develop in channels having certain variable combinations, while meanders occur in different conditions; 

the straight pattern can occur in either. 

Although causes for pattern changes cannot be 

precisely delineated (Morisawa 1968), channel 

changes obviously take place in response to changed 

stream conditions. For the land manager, meander and 

braiding processes are usually costly because valuable 

land may erode or other investments, such as roads or 

buildings, may be destroyed. It is therefore desirable to 

control lateral stream movement, but vertical bed 

adjustment processes may be initiated if the lateral 

process is stopped. Unless durable bed armor prevents 

channel depth changes or these changes are acceptable, additional bed control measures will be required. 

If depth changes are permissible, it should be recognized bed adjustment will also take place in the 

tributary streams. Thus a stream control measure may 

have adverse effects far beyond the immediate region 

of the stream. 

As a general rule, the land manager should live with 

the present natural adjustment processes and, if possible, offset losses with land improvements elsewhere. 

Channel Shapes 

Channel shape is of management interest because 

shape is predominantly influenced by the (1) quantity of 

water; (2) the type of sediment load (suspended load, 

the material moving in suspension; or bedload, the 

coarse material moving on or near the bed); and (3) the 

type of bank material. Any change in one or more of 

these factors will therefore introduce a shape change 

as well. Recognition of shape changes is a first step in 

the determination of their cause, but recognition is not 

always easy because of turbidity of the water, or transitional shape changes that are not easily seen (for 

instance, from slightly parabolic to rectangular). For 

these cases, the width-depth ratio and the shape factor 

can be calculated from field survey notes on channel 

cross sections. Both will be described in succeeding 

sections. 

Land management measures can influence water 

and sediment discharge. Costly stream training 

measures such as riprap or gabions change banks and 
their material makeup. Such changes may be beneficial 

or detrimental to management goals. For example, a 

semicircular cross section, not existing in natural 

streams, would convey the largest quantity of water 

with the least resistance (erosion or deposition). The 

more channels deviate from this idealistic shape, the 

less water will be conveyed, and flow resistance increases. Thus a wide flat channel will show tendencies 

for aggradation by sediment, possibly leading to a 

braided channel pattern. Obviously, granting permission for additional water conveyance through such a 

channel would add to the problem. 

Relations between shape and sediment load show 

channel width increases with increasing coarseness of 

the sediment particles. Thus, causes for channel 

widening may be traced back to new sediment sources 

on the watershed caused by events such as landslides 

or rockfalls from talus (cliff debris) slopes, or channel 

deepening in an upstream reach may make available 

coarser material. Of course, the basic question of what 

caused slides or deep cutting must still be answered 

before corrective management actions can be initiated. 

Also, the particle sizes in the channel banks determine channel shape. While banks with a high silt-clay 

percentage lead to narrow and deep channels, those 

with a predominance of sand and coarser material 

develop into wide, flat channels. Thus, lateral stream 

movements may lead to shape changes by exposing different bank materials. 

Lateral stream movement may not be caused by 

management but may result from natural adjustment 

processes where dynamic equilibrium was lost due to 

unusual events such as earthquakes or land flows 

(unusually large land slides). If bank material changes 

are detected at an early stage and future problems are 

expected, protective actions could be taken in time. For 

such a decision, projections must be made as to other 

types of adjustment the stream may undergo if lateral 

movement is stopped. Down cutting may take place, for 

example, requiring additional bed control structures 

under expected severe conditions. 

Width-depth ratio.-The width-depth ratio relates 

the top width (W) of the channel to the mean depth (d). 

Mean depth is defined as the cross sectional area at 

bank-full stage divided by the top width. Where bankfull stage would not be a meaningful criterion, channel 

width and flow depth should be taken at high water 

mark. In cross sections with nongeometric forms, a 

planimeter or similar instrument is best suited for 

determining cross-sectional area. 

In cross sections with pure geometric shapes, the 

following relationships hold: in a triangular channel, 

the width-depth ratio remains constant with changing 

discharge; the width-depth ratio decreases in trapezoidal and elliptical cross sections when discharge 

increases; and width-depth ratio decreases much more 

rapidly with increasing discharge in rectangular 

channels. 

Width-depth ratio changes with discharge can be 

graphically shown if the ratio is expressed as a simple 

power function of discharge: 

wld = rQs 

where Q is the discharge, and r and s are numerical 

coefficients. The value of s, the slope of the line resulting from a log-log plot of the equation, expresses 

the relative rate of decrease of the ratio with increasing 

discharge (i.e., it constitutes a measure of how readily 

a stream adjusts width and depth to discharge 

changes). An example from three stations is plotted in 

figure 18. 

In general, cross sections in straight reaches have a 

large width-depth ratio. In bends, this ratio is small. 

The sharper the bend, the deeper the pool, and the 

smaller the ratio (fig. 7). This relationship is so strong 

the cross-sectional shape of the pool could be used to 

determine the approximate degree of curvature of the 

bend if empirical data on bends and bank material are 

available. 

Schumm (1960) found that in western channels the 

type of material in banks and bottoms controls the 

cross-sectional channel shape. When the mechanical 

analysis of the soils was related to the width-depth 

ratio, linear regression indicated that increases in 

the ratio conformed with the increases of the average 

percent sand in the measured load: wide, shallow 

channels form in sandy soils, while clayey soils lead to 

narrow, deep channels. 

An extremely large width-depth ratio for a given 

river would indicate braiding must be expected 

because of large available loads and unstable banks or 

very large width of channel. The decreased flow will 

concentrate into smaller branches to obtain some 

depth and velocity for the transport of the material. 

Values for the ratio should be evaluated relative to 

prevailing values within the stream system. The range 

of the ratio can be large. Fahnestock (1963) found 

values ranging from 10 to 71 for glacial streams of 

Mount Rainier in Washington, while those for small 

streams in the southern Rocky Mountains of Colorado 

ranged between 3 and 27 (Heede 1972). 

Shape factor.-The shape factor is the quotient of 

maximum depth divided by mean depth. Because the 

value may be the same for a variety of unusual cross 

sections, the factor must be interpreted cautiously. 

Only in exceptional cases, however, do channels in 

alluvial material have unusual sections. If considered 

as geometric figures, a triangular channel has a shape 

factor of 2, a parabolic section 1.5, and a rectangular 

section 1.0. 

As stated before, the semicircular channel would 

have the most efficient cross section, because for a 

given cross-sectional area it has the smallest wetted 

perimeter. If slope and roughness are constant, the 

velocity increases with the hydraulic radius, which is 

the coefficient of area of flow over wetted perimeter. 

Hence, the semicircular channel would discharge more 
water than any other' channel shape. The most efficient 

naturally occurring cross section is a parabolic channel with a shape factor of 1.5. Thus, the discharge 

efficiency of a channel can be quantitatively judged. 

This may be of importance for projects which require 

additional conveyance of water. Where cross sections 

appear to be obscure, plotting the section will reveal if 

the factor truly expresses the shape. In the streams 

mentioned above, Fahnestock's (1963) shape factors 

ranged from 1.1 to 2.7 and Heede's (1972) ranged from 

1.1 to 2.3. 

Avoiding pitfalls in "in-stream flow" investigations.- 

Surveys of channel cross sections are an integral part 

of the land manager's "in-stream flow" investigations. 

The present type of flow should be defined because, as 

shown in a previous section, with flow the thalweg may 

Discharge (cubic feet/second) 

Figure 18.-Relationship of width-depth ratio to discharge for 

three stream stations of Fool Creek, central Rocky Mountains 

in Colorado. Station A adjusted most quickly to discharge 

changes (sediment carrying capacity is better maintained in A 

than in B or C). 

Figure 19.-Schematic diagram of a channel cross section in an 

anabranching river. The overall appearance suggests several independent parallel streams. 

change its position and in turn channel shape. At low 

flow, for example, the stream may create a pool at a 

bank due to increased flow sinuosity; while at high 

flow, water fills the channel and a low occurs in the 

section center. 

How flow types can influence channel shapes is 

determined not only by the thalweg position in the 

cross section, but also by changes of the stream power. 

The cross sectional channel shape is directly related to 

the changes of the stream power TV across the stream, 

where T is the tractive force yRS; y represents the 

specific weight of water, R is the hydraulic radius Alp, 

A represents the area of flow, p is the wetted 

perimeter, and S is the slope. The wetted perimeter is 

that part of the channel cross section submerged under 

the water of a given flow. Simons and Richardson 

(1966) have shown TV is related to mean sediment grain 

size, which determines bedform and, in turn, resistance to flow. 

Cross sections can be influenced also by channel patterns. Thus a straight reach tends to have a V-shaped 

cross section; in bends, a pool exists at the outside 

bank (fig. 7); while in braided streams the cross section 

has an undulating outline. Where anabranching takes 

place, a given stream location may have a succession of 

channel cross sections each separated from the next 

by unchannelized sections (fig. 19). The survey should 

therefore include information on the channel pattern 

and the location of the section relative to pattern 

features such as "at crossover between meanders." 

This information is important because the pattern itself 

may be responsible for a given type of flow and channel 

shape, and different processes induced by different 

patterns may form similar cross sectional shapes. 

Examples will illustrate this. 

In a straight reach, two circulation cells have been 

observed in the plane perpendicular to the flow 

(Leopold et al. 1964). The authors presented crosschannel water-surface profiles showing a marked central hump and water surface elevations lower at the 

two edges of the stream, resulting from the transverse 

water circulations (fig. 20). This so-called secondary 

circulation moves from the edges to the stream center 

and then downward. There may also be several circulations above each other, but circulation seems to 

take place in an even number of cells (Koloseus 1971). 

The secondary circulation has been associated with 
turbulent flow in prismatic channels illustrating an 

influence of channel shape on flow characteristics. 

At a meander crossing (the point of inflection from 

one meander to the next), the flow is often deflected 

towards one bank. This leads to the formation of a pool 

there and a bar at the opposite bank. Appearance of 

such a section in a cross-sectional drawing may be 

similar to that of a meander. Only the plane view would 

show dissimilarity in appearance. 

If instability of flow increases at a meander crossing, 

a bar may form in the middle of the channel, practically dividing the flow into two branches. At low flow, 

this may appear like the cross section of a braiding 

stream. Description of the section location is therefore 

required. Because of inherent instability, meander 

crossings should be avoided for structural installations 

such as bridges and culverts. 

Channel Banks 

With time, most natural channels form their bed 

and are therefore embedded in alluvial materials. 

Banks may offer serious soil mechanical and fluvial 

geomorphologic problems. Factors such as temperature, chemistry of clay, or vegetation influence bank 

stability. Bank material may range in size from clay to 

boulders, and the occurrence of this material may 

change with location or it may be stratified. Generally, 

banks with high clay content are more stable than 

those without clay. Therefore, the bank material must 

be considered if bank stability is evaluated. 

Failures of banks are customarily classified into slip 

failure and scaling. Scaling is a type of exfoliation that 

produces thin flakes, laminae, or scales. Slip failures 

are induced by piping (development of pipe-like subterranean tunnels) or often indirectly by horizontal 

stratification of the bank materials. Scaling begins 

in smaller magnitudes at the bank toe, generally, and 

proceeds up the bank. 

Where piping takes place, material stratification 

may add to the problem. This stratification should be 

considered relative to high and low flows and to the 

elevation of the water table in the bank. Piping is subFigure 20.-Idealized secondary circulation cells in a cross section of a straight reach. Water surface elevation shown (Leopold 

et al. 1964). Note the water surface shows a hump in the channel center and a low at both banks. The central hump may be as 

much as 1 foot above water surface at banks. 

terranean erosion where soil is removed from root 

canals, cracks, burrows, or other voids in the solum. 

Normally, pipes have an inlet and outlet, the latter 

at a lower elevation, and thus convey overland flow. 

Repeated flows increase tunnel size until it collapses. 

Heede (1971) described soil piping in gully banks 

with high-clay soils (>45% clay), and found piping was 

related to high exchangeable sodium percentage, low 

gypsum content, and fine-textured soils with montmorillonite clay. 

Vegetation as a bank stability factor must be 

evaluated in terms of plant vigor, density, and rooting 

depth. Obviously, the stronger, these characteristics, 

the greater the impact on bank stability. Research 

results are few on this subject, and the designer can be 

guided only by experience and judgment. 

Where the load of a stream consists of large amounts 

of fine sediment, extensive berms may form along the 

banks by deposition of silts and clays. On the other 

hand, where channels widen continuously, the process 

must be suspected to be a function of the bank material. 

Processes Affecting Longitudinal Profile 

Longitudinal Profile Relationships 

The longitudinal profile of a stream is determined 

largely by the topography. In general, stream gradients 

will decrease from the headwater downstream to the 

mouth, but gradient steepening or lowering may occur 

in any stream reach, especially if there are human or 

natural controls such as rock outcrops or rock type 

changes. When evaluating the overall shape of the 

longitudinal profile, one must be aware of the scale 

factor that may enhance local irregularities and thus 

misconstrue the overall shape. Profile shapes can be 

classed as concave, straight, or convex if local irregularities caused by bed features such as bars, pools, 

and riffles are ignored. 

The longitudinal profile is a function of the following 

variables: discharge, load, size of sediment particles, 

flow resistance, velocity, width, depth, and slope. Since 

there are more unknowns than equations, an exact 

solution incorporating all variables is not possible. 

Simplified equations of various forms have been devised, however, typified by two kinds. One is based on 

Sternberg's (1875) abrasion law, the other on Gilbert's 

(1880) law of declivities. 

Langbein (1964) suggested the concavity of river 

profiles with uniform discharge increases with length 

and decreases with rate of discharge. He proposed 

further that profile shape is independent of base level 

and is determined by length, fall, and discharge. Concavity can be expressed as the ratio 2A'IH, where A' is 

the difference in elevation between the profile at middistance and the straight line connecting the ends of 

the profile, and H is the total fall (fig. 21). 

In small mountain watersheds with homogeneous 

geology, Heede (1977) found ephemeral streams with 

convex profiles, while perennial streams showed con- 
cave profiles. He argued, considering homogeneous 

geology and other parameters of flow and channel 

geometry, concavity represents the equilibrium profile 

and convexity represents the profile of streams outside 

of equilibrium. 

Previous sections showed that the processes leading 

to a given profile are interrelated with others. Together, the processes tend to either establish or maintain equilibrium conditions. Since the profile directly 

influences flow velocities, artificial changes of the 

equilibrium slope should be avoided. 

Where a stream must share a narrow valley bottom 

with a road, the stream reach is often shortened, 

steepening the channel slope. Normally, a bedscarp 

develops at the downstream end of the shortened reach 

and proceeds upstream with time, deepening the channel (fig. 22) and the tributaries. The deepening of the 

main channel lowers the local base level of tributaries 

to which they must adjust. In many cases, available 

space does not permit relocating the road in such a 

way that new channel length equals the original. Artificial armor plating or check dams would solve the 

problem, but are expensive. Where check dams are 

used, it is important that the last upstream structure 

will not create a pronounced break in gradient at the 

upstream toe of the future dam deposits. A smooth 

transition from the lower to the upper gradient is required to avoid the development of a bedscarp at the 

deposition toe. An expected deposition gradient should 

be estimated from measurements of deposits above 

artificial barriers within the stream system. 

Aggradation 

If the sediment inflow into a reach is in excess of 

the reach's carrying capacity, the excess material is 

deposited until a new slope is established that equals 

the upstream slope. This is the new equilibrium slope. 

Based on flume experiments, Suryanarayana (1969) 

reported on the mechanics of aggradation. The new 

equilibrium slope established by aggradation can carry 

all the incoming sediment, but downstream the slope 

has not adjusted yet and deposition occurs. This obstructs the flow, and deposition occurs above also. 

Thus, deposition takes place above and below this 

Distance 

Figure 21.-Schematic illustration of Langbein's (1954) profile 

concavity ratio. 

location, raising the bed parallel to the new equilibrium 

slope. The rate of this movement decreases with time 

or with downstream advance, and so does the rate of 

aggradation. The channel upstream and downstream 

from the aggrading front behave as two different 

reaches with different flow conditions. Aggradation 

ceases once all slope segments allow equilibrium sediment discharge. 

Aggradation creates a slightly convex longitudinal 

profile (Suryanarayana 1969) resulting from interaction between transport and deposition. Since more 

power is required to move coarse material than fine 

material, the coarser sediment will be deposited first 

while the fines move farther downstream. Particle 

sizes of sediment therefore decrease downstream 

within the aggrading reach. Particle abrasion becomes 

an additional factor (Schurnrn and Stevens 1973). 

Where aggradation takes place, the streambed rises 

slowly and, with time, the tendency increases for the 

flow to spill over the banks. This will lead to rising of 

the banks. A good example is the Rio Grande at Albuquerque, N. Mex., where the streambed aggraded to an 

elevation 4 to 6 feet above the river's flood plain. The 

banks rose, but not high enough to contain exceptional 

floods and stream training measures were required to 

safeguard the city. 

The process leading to natural levee formation (raised 

banks) can be explained by sedimentation. When floodwaters spill over banks, a sudden loss of transporting 

power occurs at the brink of the banks due to decreasing flow depth and flow velocity. Most of the 

transported sediment is thus deposited at the brink and 

much less material is available for deposition on the 

flood plain (fig. 23). Under extreme flooding or bed aggradation, the stream may divide itself by building 

another channel on the flood-plain side of the levee, 

and the levee keeps both channels apart. This process 

may continue during successive floods, and the end 

result is a "braided" river. This example illustrates an 

important fact in stream hydraulics: different types of 

processes may lead to similar end results. 

Unfortunately, flume studies are not helpful in 

predicting the behavior of natural alluvial channels. 

The main reasons are flumes have rigid boundaries, 

and sediment characteristics such as shape and 

smoothness cannot be modeled. Knowledge of natural 

streams must therefore be developed under field conditions. Yet, due to the intricacy of different adjustment 

processes in alluvial channels all of which may be 

acting at one time, we cannot precisely forecast what 

will happen at any one point. The problem is compounded by the fact individual processes require different time scales. Thus, bed forms may adjust quickly 

and width slowly or erratically, while the processes 

leading to longitudinal stream profile changes are 

slow. The aggradation process is somewhat faster than 

degradation; the two processes generally alternate 

along the length of the stream. In short, morphologic 

adjustment processes are stochastic in nature, and 

stream behavior prediction is therefore a probabilistic 

problem. 
Figure 22.-(a) In mountain country, highway construction often infringes on streams. A relatively large meander bend (A) of Sycamore Creek, central Arizona (flowing from left to right), 

was cut off and the streambed relocated to the opposite side of the road (B). Thus 

streamlength was shortened, slope gradient steepened, and a nickpoint introduced at (D) 

that led tothe development of a bedscarp in the volcanic bed rocks. This scarp advanced 

about 300 feet uostream (C) within aaoroximatelv 25 vears. At ID), the original meander bend 

connected with short straight reach still usedby the creek. (bjkloseup view of the &foot. 

deep bedscarp (arrow). Adjustment processes not only caused a bedscarp but also tried to 

reestablish the meander, endangering the road bed by undercutting. (c) Rock riprap bank 

protection was required between A and B to protect the road. Arrow signifies bedscarp 

location. 

Unless an unusual event occurs such as a flood, slow 

profile adjustments are difficult to observe. Normally, 

many years of investigation will be required, therefore, 

to test the effectiveness of a design. Observation 

reaches should be as long and as uniform as possible so 

that the average bed conditions are insensitive to 

seasonal changes [Gessler 1971) that could distort the 

results. 

Degradation 

In general, degradation processes are extremely 

slow, especially where geologic processes such as 

uplift are involved. A classic example is the Grand Canyon in Arizona. 

Where equilibrium conditions have been violently 

disturbed (massive land slides, earthquakes), degradation may be fast initially, but will become progressively 

slower with time. An example is Manti Creek, draining 

the Wasatch monocline in Utah. In 1974, the third 

largest modern landflow in the United States lifted portions of the streambed by about 110 feet. Within 2 to 3 

years, degradation in the uplifted reach amounted to 

about 60 feet. Degradation is now so slow that downcutting rates cannot yet be established.' Manti Creek's 

behavior, typical for alluvial streams, illustrates that 

degradation processes are asymptotic in nature and, 

therefore, slow if considered over the full adjustment 

time. 

The profile of a degrading bed is concave (Suryanarayana 19693, and the associated channel cross sections tend to be V-shaped (Gessler 1971). As for 

aggradation, the processes shaping the profile of 

degrading reaches can be described in terms of sediment transport. During degradation, material is picked 

up from the bed until load limits (threshold values for 

the transport of particular grain size) are reached. 

While certain smaller sizes are still set in motion, the 

larger ones remain in place until even the small sizes 

cannot be picked up. With the limitation in bed particle 

movement, the slope of the concave profile decreases 

downstream. The gentler bed gradient will have an 

equilibrium sediment transport rate less than that of 

the upstream degrading reach. 

The tendency for formation of V-shaped cross sections in degrading channels is a result of variations in 

resistance to flow across the channel. Normally, carrying capacity for sediment is lower near banks than in 

the channel center due to bank roughness. Pickup of 

bed material may therefore increase toward the 

center. Other processes, however, such as spiral-like 

flow in bends (discussed in an earlier section) may have 

an overriding influence on channel cross-sectional 

shape. Also, the profile may not be concave if only 

considered over short distances. 

The processes of degradation as well as aggradation 

are thus correlated with flow and sediment transport. 

The aim of these processes is to adjust to some change 

'Oral communication with Manti-LaSal National Forest personnel, and personal site observations. 

Figure 24.-Tractor excavation of sediment from this streambed 

was unavoidable because of the road location. Since sediment 

loads are very high during high flows, as indicated by high frequency of large bars in the river, this branch channel will refill. 

In the long run, relocation of the road may be less costly than 

repeated excavations. 

in the stream system; the end result is a new equilibrium condition. Except for adjustment processes 

that are detrimental to immediate management goals 

or to important installations, the land manager 

therefore should not interfere. If possible, he should 

work with the stream processes, not against them. 

Such an approach is less costly. For example, dredging 

the river bottom in aggrading reaches is not always 

unavoidable, but is nearly always of infinite duration 

and excessive cost (fig. 24). 

Armoring 

In many situations, the process of degradation is 

halted by the selective transport of the flow before the 

equilibrium slope is attained. This slope would have 

corresponded to the original bed material gradation. 

An example is Livesey's (1963) report on the Missouri 

River below Fort Randall, where 15 feet of degradation 

was expected. But after the bed had lowered 3.5 feet, 

degradation ceased because an armor was established 

on the bed. Within 10 years, the Dss (65% of the 

material on the cumulative curve is finer than the size 

Dss) increased from 0.20 mm to nearly 1 m, representing 

a coarsening of bed particles by 500%. 

During the armoring processes, smaller particles are 

carried away by the flow, while the larger ones remain 

in place and create a stable bed. Two prerequisites 

must be fulfilled before an armor can be established: 

bed material must include a gradation.from finer to 

larger grain sizes (which is true for most natural 

streams), and fluctuations in flow magnitude must be 

mild and flow velocities relatively low during time 

periods long enough for armor formation. 

The armor coat is determined by the largest flows of 

the establishment period because particles light 

enough and available for transport are moved out by 

these flows, but by no means are all smaller grain sizes 

removed, although the large components are predomi- 
nant in the armor. The individual larger grains exert a 

shingle effect by fully or partially covering smaller 

ones thus keeping them in place (Lane and Carlson 

1953). There is no distinct boundary, therefore, between the armor and the sublayer. The armor becomes 

effective when it is one grain size thick (Lane and 

Carlson 1953). Future flows, larger than those experienced during the establishment of a given armor, 

may destroy the present coat and a new armor, consisting of a predominance of still larger particles, 

may be created. In contrast to the aggradation and 

degradation processes, armoring does not cause bed 

slope changes; instead, the bed is lowered parallel to 

its original slope (Gessler 1970, 1971). 

Although precise quantitative projections on armor 

development are not possible (Little and Mayer 1976), 

the investigator should attempt to determine the availability of large sediment particles in sufficient quantities which can resist the largest flows in a given time 

period. Under conditions of degradation, this material 

must be exposed by a flow before it can act as armor. 

The size distribution of the sediment buried below the 

bed layer or in the banks is therefore important. If 

newly exposed banks are sufficiently soft for alignment 

changes to take place, degradation will not continue 

generally. 

Where high values are at stake, Gessler's (1967) 

method of prediction should be used. This requires 

data on flow and sediment transport, which in many 

cases must be generated before analysis can proceed. 

Where the situation permits and data are not available, the expected depth of degradation and armor 

plating can be estimated by comparing present bed 

materials with those expected to be exposed by 

degradation. Drilling or other excavations would be required to determine the size of the lower bed materials. 

Because the judgment requires experience, an expert 

should be consulted. 

THE NEED TO MONITOR STREAM BEHAVIOR 

We should not underestimate the benefits that can 

be derived from predicting stream behavior. If bridge 

piers will be undermined by scour [degradation) or the 

bridge buried by sediments [aggradation), such knowledge is most valuable for management action even if it 

is not precise in time and volume. But data are required 

to make sound predictions. Many stream projects have 

been completed, but post-project behavior is seldom 

monitored. Monitoring is also needed because most 

theoretical relationships were developed under controlled laboratory conditions, and verification or 

modification of the relationships by case histories is 

required. Observations spanning long stream reaches 

and long time periods are expensive, but how else can 

we increase our knowledge abdut the real world? 

Monitoring should be based on sampling at random 

or predetermined stations to reduce expenditures. 

Generally, the latter approach is preferable because 

fewer stations will be required for representation. An 

example would be the establishment of one sediment 

sampling station each in the head water, middle, and 

lower stream reach, since stream gradients and sediment particle sizes generally decrease downstream 

while flow and velocity increase. Often, average gradients of the reaches can be determined from maps. 

But sampling of total sediment load, one of the most 

influential hydraulic variables, is difficult. Bedload 

can be sampled only in catch basins or where turbulent 

flow places the total load in suspension. The latter condition may be found where channels are excessively 

constricted. Thus, usually only suspended load is 

monitored (by use of dip or pumping samples) to represent sediment load. Often, suspended load is 75% to 

90% of the total. 

Bed material particle sizes can be sampled at low 

flow. A grid pattern should be used comprising an area 

on the bed from bank to bank. Sampling total bed width 

will ensure the particle changes with changing flow 

lines across the channel will be representative. 

Wolman (1954) tested such a pattern and found significant representation of the particle size distribution. 

Changes in particle sizes are indicative of changes in 

bedload transport. 

Monitoring bed material at a stream-gaging station 

will not show true sediment production in a watershed 

because of the lag time between production at a location and transport to the station. Especially in 

ephemeral streams, many years may pass before a 

substantial part of the sediment production reaches 

the station; and some amount never will (Heede 1976). 

Also, channel storage by bar and other depositional 

features (bed aggradation, riffles) generally is many 

times larger than the bedload caught at the gaging 

station. 

If any sediment transport equations are to be used, 

they should be thoroughly tested for applicability. 

Because stream behavior involves intricate processes as 

demonstrated in the preceding sections, the processes 

have different modes at different flow conditions. 

If stream-gaging stations are not available, a current 

meter or velocity headrod should be used for flow determinations. Heede (1974) showed velocity measurements by the rod require less time. In boulder-strewn 

streams, results did not differ significantly from the 

current meter readings. 

Where channel cross sections must be sampled 

because of expected severe consequences from future 

channel shape changes, it is important to select representative sections of a reach. These can best be determined from aerial photographs, if low-flying images 

are available, or by ocular selection on the ground. 

To facilitate ground surveys, bench marks should be 

established at some distance from the brink of the 

stream banks. Regular land survey tools such as a hand 

level or engineer's level will suffice. 

The intensity of stream sampling depends on the 

objective and on available funds. But if the user 

focuses on the basic stream processes as outlined in 

this report, the complexity of stream dynamics can be 
better understood and efforts limited to essential 

monitoring. 

CONCLUSIONS 

This report focuses on the major processes of stream 

dynamics and shows how the complexity of stream 

behavior forces the user to consider the individual 

components of each case. The report should be helpful 

in delineating the individual characteristics. Above all, 

stream restraining measures must be applied with 

great caution so that treatment of one critical location 

will not simply lead to the formation of another, or that 

future treatment "side effects" will not be more 

detrimental to the attainment of a new equilibrium condition than no treatment. 

From the preceding sections, it follows that the 

science of fluvial hydraulics has not been developed to 

a level that permits accurate prediction of stream 

behavior. Judgment is still required in addition to 

mathematical-statistical analyses. Yet some qualitative 

predictions are possible since conceptual relationships 

have been developed for most hydraulic interactions. If 

streamflow records are available, the investigator can 

predict at least the trend of future stream behavior. 

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NOTATION 

A = cross-sectional area of flow-square feet 

A' = difference in altitude between concave profile 

at mid-distance and straight line connecting the 

ends of profile-feet 

D = sediment particle size-millimeters 

d = depth of flow-feet 

Fr = Froude number 

g = acceleration due to gravity-feet per second 

per second 

H =total fall of stream-feet 

K, = coefficient relating sediment load and water 

discharge 

m =mass of water-pounds 

p =wetted perimeter-feet 

Q = stream discharge-cubic feet per second 

Q, = sediment discharge-pounds 

Q, =water discharge-cubic feet per second 

Re =Reynolds number 

R = hydraulic radius-feet 

r = numerical constant 

S = slope-feet rise or fall divided by feet horizontal distance 

s = numerical constant 

V = average velocity-feet per second 

w = top width of channel-feet 

y =specific weight of water-pounds per cubic foot 

p = absolute (dynamic) viscosity-pound-seconds 

per square foot 

v =kinematic viscosity-square feet per second 

Q =fluid density-pounds per cubic foot 

T =tractive force-pounds per square foot 

Agriculture-CSU, Fort Collins