### 97 

## Sediment Transport and Channel Changes in an Aggrading Stream in the Puget Lowland, Washington
Mary Ann Madej

ABSTRACT Recent land-use changes increased sediment yield in a fourth-order stream in western Washington from 22 t/km 2per year to 185 t/km 2 per year. In response, channel width increased and depth decreased, but channel gradient and mean flow velocity remained about constant. As a result of channel changes, sediment transport rates increased from 500 t/year to 4200 t/year. Active sediment constitutes 5 percent of stored sediment, and much sediment will be flushed from the system in about 20 to 40 years.

Mary Ann Madej, Geologist, National Park Service, Redwood Na

tional Park, Arcata, California 95521.

### 98 

## INTRODUCTION The sediment load of a stream is determined by such charac

teristics of a drainage basin as lithology, vegetation, pre

cipitation, and land use, and it enters the stream system by a variety of erosional processes. If stability is,to be main

tained, an equilibrium must exist between sediment entering the stream and sediment transported through the channel. Changes in sediment load can upset this balance and result in physical and biological changes in the stream system. The purpose of this study is to determine the source and volume of sediment introduced into a stream, and its subse

quent distribution, rate of movement, and influence on chan

nel geometry. Three approaches were used. First, a sediment budget quantifies the amount of sediment introduced into the stream channel. Second, sediment-transport equations predict how much sediment moves under conditions of various channel morphologies. Third, survey measurements indicate the volume and distribution of the deposited sedi

ment.


## FIELD AREA Big Beef Creek, a gravel-bedded stream, was selected for study because high rates of sedimentation and channel instability during the last 10 years were evident and baseline data were available from records kept at the University of Washington Fisheries Research Station at the mouth of the creek. This fourth-order stream drains an area of 38 km 2 on the Kitsap Peninsula in Puget Sound, western Wash

ington (fig. 1). More than 85 percent of the drainage basin

### 99 is forested with second-growth western red cedar ( Thuja plicata Donn ex D. Don), Douglas-fir ( Pseudotsuga menzie

sii(Mirb.) Franco), and associated species. The climate is characterized by cool, dry summers and mild, wet winters. Annual precipitation (predominantly rain) ranges from 1300 to 1700 mm, and most major storms occur between Decem

ber and March. The dominant surficial geologic unit in the drainage basin dates from the Vashon (late Wisconsin) glaciation. A mantle of compact till, composed of a bluish-gray mixture of cobbles, pebbles, and silt and clay, covers much of the area. In addition, poorly sorted recessional outwash sands and gravels form many of the surface deposits in the Big Beef Creek drainage basin. The upper 15 km 2 of the Big Beef Creek basin has a low channel gradient (0.2 percent), an average hillslope gradi

ent of 5 percent, and drains into an artificial lake. Because the lake acts as a sediment trap for suspended and bedload particles from the upper basin and most runoff and sedi

ment discharge are generated below the lake, this study only considers a sediment budget for the lower drainage basin, where average channel gradient is 1 percent and hillslope gradients range from 10 to 70 percent. Drainage density be

low the lake is 2.5 km/km 2 , as measured on 1:12,000 aerial photographs. In 1969, the U.S. Geological Survey established a gaging station 1.6 km above the mouth of Big Beef Creek. The fre

quency of annual peak discharges was compiled from daily discharge records. The 10- and 50-year floods are about 30 and 56 m 3 /second, respectively. No flood has exceeded 30

m 3 /second in the past 10 years, and regional records indicate that no 50-year floods have occurred in the last 25 years. Thus recent changes in the Big Beef Creek basin occurred under fairly normal climatic patterns. The principal activities in the drainage basin since the 1850’s have been logging and some cattle grazing. Most vir

gin timber was logged by 1930 and a resurgence of loggingactivity occurred in the mid-1950’s, when harvest of secondgrowth timber was initiated. Urban development is becom

ing widespread and this decreases the amount of forested land as well. In 1942, 3 percent of the land was deforested, by 1965 the figure was 12 percent, and today it is almost 20 percent. Likewise, road density tripled, from 1.2 km/km 2 in 1942 to 3.5 km/km 2 at present. Personnel at the University of Washington Fishery Research Station noted heavy sediment deposition at the mouth of Big Beef Creek during the winters of 1966-67 and 1967-68; in the summer of 1969, the lower 500 m of the creek were channelized artificially. In 1970, a weir 2 m high was built across the mouth of the stream, which has acted as a small dam and has caused deposition of bed material for several hundred meters upstream of the weir. 

## METHODS Cross-Section Surveys Fifty-five cross sections along the main channel of Big Beef Creek were surveyed periodically from 1969-78 by Ceder

holm (1972) and me (fig. 1). Gradients of channel thalweg and water surface also were determined in several locations. Detailed records of channel changes at one cross section were obtained from U.S. Geological Survey gaging station records for 1969-79. These records (Forms 9-207 and 9-275) consist of bimonthly measurements of width, cross-sectional area, mean velocity, discharge, and stage relative to gage datum. 

### Regional Channel Geometry Survey Because the channel geometry of Big Beef Creek was not determined until after the drainage basin had been disturbed by road construction and timber harvest, predisturbance geometry was predicted from a regional channel-geometry relation developed from relatively undisturbed drainage ba

sins in similar geologic and geographic settings. Estimates of regional channel geometry were obtained by measuringbankfull width and depth, channel gradient, and sedimentsize distribution and correlating them with drainage area at 20 sites on nearby streams (Madej 1978). Comparison of di

mensions of the present Big Beef Creek channel with those from undisturbed channels draining the same area indicates the extent of changes in Big Beef Creek.


### Sediment Studies Size distribution of sediment in the main channel and tributaries was characterized with the pebble-count method described by Wolman (1954). Samples of channel gravel, and hilislope and bank material were also collected, sieved, and weighed. Sediment distribution within the main valley of the drainage basin was determined by mapping the location and volume of gravel bars, overbank sand deposits, terraces, tributary deltas, and debris fans. Aerial photographs at a 1:12,000 scale from seven flights between 1942 and 1976 allowed me to compare the locations and sizes of large gravel bars, log jams, and braided reaches through time. Area and location of mass movements were also mapped from photographs. Field checking provided more accurate estimates of volumes of material, as well as ages of mass-movement features dated by vegetation. The size of the largest particle of bed material moved from a site by high flows during 1977 was obtained by observ

ing marked rocks. Over 2,000 painted rocks were scattered on the upstream end of six gravel bars in the main channel and tributaries. The painted rocks ranged from 16 to 180 mm (D50 = 64 mm). After high flows, the size of each rock found and the distance it had moved were recorded.

### 100 Bedload-transport rates were estimated by the methods proposed by Einstein (1950), Meyer-Peter and Muller (1948), and Emmett (1976). Dr: H. W. Shen of Colorado State University supplied a computer program of the Einstein BedLoad Function. This program was used to compute bedmaterial load under various conditions representative of the changing channel geometry from 1969 to 1978.


## RESULTS
Measurements Table 1 lists the results of cross-sectional surveys of six reaches during three periods. Generally, aggradation oc

curred in lower reaches, degradation in middle reaches, and little change occurred in upper reaches, as illustrated in selected cross sections (fig. 2). Net aggradation occurred in Reach 1 above the weir. Be

tween 1969 and 1976, the bed aggraded 0.5-1 m in the lower 300 m. Overbank flow deposited material over an area 70 m wide and built small levees to form new banks about 0.5 m above old banks. Alders rapidly revegetated overbank de

posits and helped stabilize new banks. Overbank deposition is active, as indicated by several centimeters of sand burying young alders. Net aggradation occurred in Reach 2 (fig. 2). The chan

nel responded by assuming a more braided pattern in this reach and forming midchannel gravel bars. Bankfull width increased about 25 percent from 1973 to 1978, and bankfull depth decreased by a similar amount. Bankfull velocity (from U.S. Geological Survey records) remained about con

stant throughout this period. Figure 2—Selected vertically exaggerated cross-sectional surveys in the main channel of Big Beef Creek.

### 101 Bank erosion and scour took place at several locations in Reaches 3 and 4, which may be a response to previous ag

gradation. Where roads are still contributing large volumes of sediment, aggradation and growth of gravel bars is ap

parent. In contrast, the channel just below the lake (Reach 5), which is located in a relatively undisturbed area, showed little change. Longitudinal surveys indicate that, in the lower 2 km, chan

nel gradient decreased from 0.090 to 0.085 between 1970 and 1977. Total channel length increased from 9785 to 10 010 m. Sinuosity increased from 1.30 to 1.32, and overall channel gradient decreased slightly. Patterns of channel length changes correlate approximately with trends of cross-sectional changes. Where little cross-sec

tional change occurred, channel length remained constant. In degrading reaches, channel length increased; in the aggrading lower reaches of Big Beef Creek, channel length decreased. The predicted and actual values of width for lower Big Beef Creek differ considerably (fig. 3), although the difference is negligible for the upper stream below the lake. Values for bankfull depth, channel gradient, and mean particle size are within limits of scatter in the regional pattern. After one winter, 53 percent of the painted rocks were found. Large particles (>128 mm) did not move as far as intermediate sizes (45-90 mm). Surprisingly, small particles ( 45 mm) did not move as far as those in the intermediate size class. Many of these small particles were found lodged behind large ones and were protected from further move

ment. Mean distance moved by intermediate-sized particles (45-90 mm) was 80 m.
Distribution of sediment stored in the channel is listed in table 2. In side channels, the volume of sediment above bedrock was measured. In the main channel, sediment was only measured to the depth of scour which was taken to be 0.4 m, the maximum depth of degradation in cross-sectional surveys. Log jams are temporary sediment traps for bed material moving downstream. Logging activity may increase the number and magnitude of log jams in a stream channel, which in turn may increase the deposition of gravel and sand. In a relatively undisturbed reach, four log jams were noted, or 2.5 jams/km of channel. In reaches with recent logging, the concentration was 12 jams/km and these jams were larger than those in undisturbed reaches. Log jams are associated locally with braided channels as the stream bends around the jams. Suspended sediment was sampled periodically at 0, 0.8, and 1.5 km above the mouth. Using U. S. Geological Survey dai

ly discharge data, I constructed three suspended-sediment rating curves (Madej 1978). Average suspended-sediment load near the mouth of Big Beef Creek is 2325 t/year (110 t/km2 per year), assuming the basin above the lake does not contribute any suspended sediment to downstream reaches. Figure 3—Results of regional channel-geometry survey for Puget Lowland Streams (present dimensions for Big Beef Creek at 1.5 km are indicated by an asterisk).

### 102 

## CALCULATIONS Sediment Budget The amount of sediment entering Big Beef Creek can be quantified by examining the contribution of processes sup

plying hillslope material to channels. Sediment input in an undisturbed area was considered to enter channels by, soil creep and hillslope failures. Rate of soil creep under edaphic and climatic conditions in the Big Beef Creek drainage ba

sin was estimated from published creep rates for other areas (table 3). Three studies have been conducted in temperate areas receiving about the same annual precipitation as Big Beef Creek (1300-1500 mm), and the coarse soil textures in these study areas are similar to the Big Beef Creek basin. Average rate of creep for the top 15-25 cm of soil for all the studies lies within a narrow range, and a value of 1.5 mm/year was chosen as a reasonable estimate of creep rate in the Big Beef Creek drainage. In addition, tree throw (Dietrich et al., this volume) contributes the creep equivalent of 1.5 mm/year on steeper, wetter hillslopes. The extent of tree throw in the Big Beef Creek basin is not as great, and I estimate 1.0 mm/year of soil movement by tree throw, or a total rate of 2.5 mm/year.

The most active layer of soil in terms of creep is the upper zone, which is influenced by such processes as root growth and decay, tree fall, and burrowing of animals. In the Big Beef Creek basin, the observed thickness of this active layer is about 0.5 m on hillslopes of 25-45 percent, and it has a density of about 1.7 t/m 3

. Total length of first- to fourth-order channels in the lower drainage basin is about 57 500 m. Because soil transported downhill by creep enters channels through shallow bank fail

ures on both sides of a channel, the total amount of material in

troduced by creep is: 5.75 x 104 m x 2 x 0.5 m x 0.0025 m/year x 1.7 t/m 3 = 245 t/year = 10.6 t/km 2 /year. This is a minimum estimate, assuming soil deeper than 0.5 m contributes little material in comparison to the more active surface layer. Material discharged through soil creep is transported either as bedload or suspended load, depending on the size of material. Suspended-sediment samples collected in Big Beef Creek have a maximum particle sizes of 1 mm. Textural analyses of soil in the Big Beef drainage basin indicate that 40 percent of the active layer is greater than 1 mm. Thus, 147 t of material discharged into the channel through soil creep become suspended load, and 98 t is transported as bedload. Some bedload may break down into particles small enough to be carried as suspended load. Tumbling experiments with rocks of similar lithology and size as material in Big Beef Creek (Kuenen 1956) show that 10 km of transport is insufficient to break down much material. Average rate of attrition is 0.1 percent/km of transport. Large discrete hillslope failures are the second major con

tributor to a stream’s sediment load. Field measurements of depth of slides averaged 0.6 m. During the last 10 years, 880

m 2 of land was affected by landslides that were not associ

ated with roads, logging, or residential lots. So, the rate of mass wasting for undisturbed parts of the basin is: 880 m 2 x 0.6 m x 1.7 t/m 3 10 years = 90 t/year. These slides, whichoccurred on slopes >15 percent or at the headcuts of firstorder channels, were representative of an area of 6 km 2in the lower drainage basin, so the rate can be expressed as (90 t/year)/(6 km 2 ) or 15 t/km 2 per year. This rate is lower than estimates from other drainage basins in wetter, steeper envi

ronments (Swanston 1969, Dietrich 1975, Pierson 1977). Because human activity significantly affected the remainder of the lower drainage basin, the rate of naturally occurring slides for the entire lower basin must be extrapolated from the rate of landsliding in the undisturbed sections. An area of 6 km 2 has hillslopes <15 percent, and probably does not contribute sediment through landsliding. The remaining 17 km 2 contributes 15 t/km 2 per year, or 255 t of sediment per year. This sediment is characterized by the size distribution of the average soil column. Thus, 153 t/year is suspended load, and the remainder becomes bedload. Consequently, in an undisturbed state, sediment input into Big Beef Creek was: Soil creep - 245 t/year 147 t/year of suspended load 98 t/year of bedload Mass movements - 255 t/year 153 t/year of suspended load 102 t/year of bedload Total load - 500 t /year 300 t/year of suspended load (22 t/long per year) 200 t/year of bedload This value for total sediment yield lies at the lower end of the range of sediment yields reported from drainages wet

ter, steeper, or both--such as 28 t/km 2 per year in western Oregon (U. S. Environmental Protection Agency 1971), 73 t/km 2 per year in southwest Alberta (Hollingshead 1971), and 105 t/km 2per year in British Columbia (Slaymaker and McPherson 1977).

### 103 The Big Beef Creek drainage basin is no longer an undis

turbed system because of changes in land use. In areas undergoing logging and road construction, rates of occur

rence of mass movement may increase (Anderson 1954, Fredriksen 1970). In addition, processes such as sheetwash and rainsplash erosion--normally insignificant in humid, well-vegetated areas--may transport material from roads to channels. Annual sediment contribution by disturbancerelated land

slides was quantified by mapping volumes of slides originat

ing on roads and cut areas, and dividing volumes by the age of slides. These slides were smaller but more numerous than naturally occurring slides on valley sides. Between 1968 and 1977, an average of 845 t/year entered the stream channel through debris slides induced by land-use changes.
Roads are an additional source of sediment. Sheetwash on roadbanks and roadbeds, erosion of roadside ditches and sidecast material, roadbank gullying, and dry ravel from roadbanks are all active on roads in the Big Beef Creek basin. The volume of material eroded from roadside ditches was estimated in the field. Alluvial fans are not found at out

lets of culverts draining ditches, and the largest painted rock moved in roadside ditches had a diameter of 64 mm. Thus, most eroded material probably reaches the stream channel. Roadside ditches contribute 440 t/year to the stream system.
Sheetwash carries sediment from roadbanks and roadbeds to stream channels. Few studies have directly measured amounts of sediment originating from roads, but these stud

ies indicate the order of magnitude of sediment eroded from unpaved roads: Hornbeck and Reinhart (1964) - 9900 t/km 2 per year for fresh roads and 125 t/km 2 for abandoned roads; Diseker and Richardson (1961) - 20 000 to 60 000 t/km 2 per year; and Wolman and Schick (1967) - 20 000 to 50 000 t/km 2 per year. L. Reid and T. Dunne (University of Wash

ington) are currently studying erosion on logging roads, and their results for moderately used and abandoned roads are of the same magnitude as those of Hornbeck and Reinhart. Of these estimates, the 9900 t/km 2 of Hornbeck and Reinhart seems the most reasonable to apply to the Big Beef Creek drainage basin, because these authors studied tractor-yard

ing skid roads under slope and precipitation conditions similar to those in the Big Beef drainage basin. Although soil loss in the above studies was measured under a variety of conditions, sediment yields obtained were always far above the sediment yield in undisturbed areas. To apply these values directly to the Big Beef Creek drainage basin is risky because of variation in local conditions. Nev

ertheless, these values are useful estimates of soil loss from roads in the Big Beef Creek drainage. Road prisms in the Big Beef Creek basin average 8.m in width. In 1977, 30 of the 65 km of unpaved roads were ac

tively used. Using Hornbeck and Reinhart’s values of sedi

ment yield, I computed soil loss on roads to be: .008 km [(30 km x 9900 t/km 2 per year) + (35 km × 125 t/km 2 per year)] or 2410 t/km 2 per year. By examining the size distribution of lag deposits of the road surface, I estimated that 80 percent of the eroded material (1930 t) was transported as suspended sediment. Scour of roadside ditches contributes an addi

tional 440 t/year. Thus a total of 2850 t/year is eroded from roads, of which 685 t is bedload.
The total sediment load of the stream is the sum of the con

tributions of natural hillslope processes and erosion process

es from disturbed areas (table 4). Although these estimates in the sediment budget are approximate, their concordance with other measurements indicates they are of the correct magnitude. The estimate of 3000 t of suspended sediment/ year compares favorably with the 2325 t/year obtained from suspended sediment rating curves. Estimated bedload input is 1200 t/year; the volume of sediment deposited in the low

est kilometer of Big Beef Creek between 1970 and 1977 was 4700 m 3 , or 9000 t, an average load of 1400 t/year. Some of the 1400 t/year may represent sediment moving out of stor

age from upstream reaches, rather than bedload input from hillslope processes.

The computed sediment budgets suggest that total load of the stream increased from 500 t/year under undisturbed conditions to over 4200 t/year at present. Even if absolute values of sediment input are contested, the apparent contrast of sediment loads under undisturbed and disturbed condi

tions are large and significant. Sediment Transport Once sediment enters a channel, it is transported down

stream during high flows. Bedload transport represents work done by a stream and is a function of the boundary shear stress generated at any given moment and place. Boundary shear stress, τ b , produced by a fluid is calculated as:

where u* is related to velocity a at distance z from the chan

nel boundary by

where ρ = density of the fluid, u
* = shear velocity, and 0.4 = von Karman’s constant. The depth at which velocity appar

-

### 104 ently goes to zero as the channel bed is approached is the roughness length of the boundary, zo, and is dependent upon particle size. It can be estimated by D65/30, where D65 is the grain diameter that is greater than 65 percent of the bed material (Graf 1971). Bedload transport rates for the gaging site at Big Beef Creek (fig. 1) before and after disturbance were calculated from Einstein (1950), Meyer-Peter and MUller (1948), and Em

mett (1976) (table 5). I have discussed their equations and the applicability to Big Beef Creek (Madej 1978). Dimen

sions of the undisturbed channel were estimated from regional channel-geometry results; those of the disturbed channel were obtained from survey measurements. Mea

sured frequencies of discharge for each year were used. The three equations predict that an average of between 600 and 1400 t of bedload/year would be transported past the gaging site.


## INTERPRETATION Present channel morphology allows greater bedload trans

port than the undisturbed channel (table 5). Analysis of boundary shear stress in the channel leads to the same conclusion. An increase in stream-channel capacity must be caused by an increase in τ b . Using U.S. Geological Survey velocity and discharge measurements, I compared magni

tude and distribution of shear stresses at four sites near the gaging station at equivalent discharges over a 4-year period (Made] 1978). Figure 4 shows the results of this analysis at one site, where erosion occurred at the right bank and the channel was 2 m wider in 1975 than in 1971.
Several common characteristics were found in the four analyses. The channel became wider and shallower through time (also shown by periodic cross-sectional surveys). Large gravel bars have low shear stresses above them and high shear stresses around them, perhaps caused by flow acceler

ating around the barrier. Most important, high shear stresses are found in areas of bank erosion, and the high stresses can be maintained over a shallower cross section. Thus, the abil

ity of the channel to transport sediment is increased under the new channel configuration. 

### Channel Changes: Possible Modifications The geometry of alluvial rivers can adjust to changing conditions imposed upon them (Mackin 1948, Leopold and Maddock 1953). In Big Beef Creek, an increase in sediment load was accommodated by changes in several interrelated hydraulic variables. Grain size and lithology of sediment supplied to a stream are variables independent of the stream system, as is discharge entering a stream. Other factors are semidependent and can undergo some modification by a stream in response to changes, i.e., flow resistance, mode of sediment transport, sediment-particle size, and channel pattern. Some parameters are almost totally functions of processes of the stream itself and can be considered to be dependent: velocity, width, depth, and channel gradient. In response to an increase in sediment load, the more dynamic factors in a stream change to accommodate the in

crease. Adjustments are limited by certain physical relation

ships, however. The first of these is the continuity equation:

where Q = discharge, w = width, a = mean depth, and u = mean velocity. The Manning equation (4) defines the relationship between a and d:

where s = slope of the stream-energy line and n = turbu

lent-friction coefficient (roughness). In addition, sediment transport, Qs, is here assumed to be dependent upon stream power: Figure 4—Distribution of boundary shear stress in main channel, 1.6 km above the mouth of Big Beef Creek.

### 105 The manner in which the Big Beef Creek channel changed is summarized in a previous section. Theoretically, however, these changes do not represent the only possible changes, and it is important to explain why the channel has been modified in this manner. Because transport of sediment.rep

resents work accomplished, the amount of sediment trans

port was used as the criterion for capacity for work among various channel configurations. The Einstein method was used to compare the transport capability of several channel configurations. Several combinations of width, depth, and velocity (re

stricted by continuity and roughness relationships) were tested with the Einstein computer program (fig. 5). An increase in width/depth ratio indicates increased bedload sediment transport (Schunm 1968, Pickup 1976). This holds true if velocity is held constant (line A in fig. 5), implying a decreasing roughness coefficient. In a second set of calculations, velocity changed. If slope and roughness are considered to be a constant, k, the value of velocity from the Manning equation is kd 2/3 , and dis

charge is defined as: Q = kwd 5/3 . In this case (line B in fig. 5), rates of sediment transport decrease with increasing width/depth ratio. Therefore a change in the roughness ‘n’ apparently is needed to increase transport rates.

Discharge measurements indicate that bankfull velocity remained about constant over the last 8 years, although the width-to-depth ratio, w/d, increased from 19 to 27. Line A in figure 5 suggests that w/d can increase indefinitely to trans

port increasingly larger sediment loads; however, in reality, stream width does not increase indefinitely. To understand what mechanism limits the increase and where these limits may be expected to occur, the effect of bed-particle rough

ness on flow must be considered. Coarse surficial particles on a river-channel bed cause turbulence and energy loss in water flowing over them. The effect of these particles is most pronounced at low flows. Leopold et al. (1964, p. 160) described this phenomenon in gravel-bedded streams using a ratio of depth of water (d) to particle size (D 84 ) of the armor layer. The relationship is described as:
where npis roughness from bed particles and g is the accel

eration from gravity, 980 cm/second 2

. Hence roughness from bed particles is dependent only on depth of water and not on width or velocity. Values of n predicted by this method are minimum values, because other forms of roughness such as gravel bars, log jams, bank roughness, and meanders increase n. But water cannot flow over a rough bed with values of n below that predicted by the d/D 84relationship (equation 6). At any given discharge, if width and depth are defined, velocity is known through the continuity equation. If veloc

ity and slope remain constant as depth changes, roughness as defined by the Manning equation must change. Figure 6 shows the relationships of width, depth, velocity, and rough

ness at bankfull discharge (10 m 3 /second) in Big Beef Creek. Because no streams in the regional survey show a w/d less than 18, only those greater than 18 are considered. Actual ratios are below 27, but hypothetical cases with higher ratios are considered as well. Because survey measurements show little change in channel gradient, gradient is assumed constant in this analysis. Manning’s definition represents total roughness, nt , contributed by several factors; the value of nt decreases with increasing width and is given for each location considered. With decreasing depth, roughness from bed particles, np , in

creases (from equation 6), and horizontal contours of figure 6 indicate the value of npat different depths of flow.

Figure 6—Velocity and roughness constraints on the Big Beef Creek channel configuration at bankfull discharge (relationships determined from equations 3, 4, and 6).

### 106 Records from the U.S. Geological Survey gaging station show that mean velocity at bankfull stage remained between 150 and 160 cm/second over an 8-year period. Only a small portion of the diagram represents channel configurations where bankfull discharge is accommodated within this velocity range (the stippled area of fig. 6). Because veloc

ity remained constant despite changes in channel form, I assumed velocity would remain constant if the width/depth ratio increased, and so the stippled area also includes hypo

thetical configurations. As long as discharge can flow with a roughness term greater than that predicted by the d/D 84relationship, the channel configuration is possible. Conversely, once bed-particle roughness is greater than the roughness needed to keep ve

locity constant, satisfying conditions of continuity, Q = wdu, becomes impossible. Thus, a hydraulic constraint prevents unlimited increases in channel width. This relationship implies that the amount of sediment that can be transported as bedload is limited. Sediment trans

port can increase with width-to-depth ratio only as long as roughness-velocity constraints are met. If all sediment sup

plied to the channel cannot be transported at the maximum possible width/depth configuration, aggradation occurs. Configurations that are not within the constant velocity envelope probably do not exist, because the amount of work necessary to create those situations is large. For example, point A in figure 4 is associated with a velocity of 213 cm/ second, and nt = 0.0195. Such a low value for total channel roughness is rarely, if ever, found in gravel-bedded streams (Barnes 1967). Alternately, point B in figure 4 represents a situation where velocity is 76 cm/second and nt = 0.087. Because np is only 0.008 at this depth, the nt value of 0.087 would be difficult to reach even with the contribution of other causes of roughness, and this configuration is improb

able. Big Beef Creek does not display the maximum theoretical width of 30 m. Several reasons may account for this. A sta

ble channel must have boundaries that can withstand shear stresses imposed by the flow. Using Graf’s method (1971), critical shear stress on channel walls in Big Beef Creek is between 55 and 150 dy/cm 2 , above which erosion occurs. These stresses are approached and probably exceeded at high flows (fig. 4); however, the resistance of banks caused by vegetation must be considered. A cover of Bermuda grass can resist stresses up to 480 dy/cm 2 (Leopold et al. 1964). The thick mat of tree and grass roots on the banks of Big Beef Creek probably inhibits bank erosion. Undercutting of banks below the root zone does occur, but not rapidly. The channel at the U.S. Geological Survey gaging station underwent the largest changes in 1973-74, when discharges were in excess of 20 m 3 /second. Maximum discharge since that time has been 13 m 3 second. More wid

ening may occur when another high discharge is reached, but it seems unlikely under present vegetation and discharge conditions that the channel will erode 15 m to reach 30 m. The valley widens near the mouth of Big Beef Creek, and channel width does reach 30 m, but this partly reflects weir and tidal action effects on channel geometry. 

### Storage of Sediment in the Channel In several surveyed as well as unsurveyed reaches, much sediment eroded from roads and recently harvested hill

slopes is obviously stored in the channel in the form of gravel bars. Field mapping indicates that the total volume of stored channel material is about 520 000 m 3 (1 092 000 t), of which 23 400 m 3 (49 200 t) is active material (i.e., in unvegetated bars down to a scour depth of 0.4 m) (table 2). Most active material lies in the lower 6.5 km, where channel braiding and log jams are prevalent. An estimate of potential residence time for active material under present conditions is obtained by dividing the amount by the bedload-transport rate after disturbance, 1000 t/year, or 50 years. An average rate of movement can be estimated by dividing channel length, 10.3 km, by 50 years, or 206 m/ year. Milhous (1975) obtained a particletransport rate of 234 m/year through bedload measurements in an undisturbed, steeper channel with coarser gravel. 

### Relation of Channel Changes to Land Use Changes in the Big Beef Creek channel can be related toland use in the drainage basin. Intense logging in the mid1950’s 2000 m above the mouth dramatically increased sediment load in that reach, and aggradation occurred. Assuming bed material moves 200 m/year, effects of excess sediment would be felt near the mouth in (2000 m)/(200 m per year), or 10 years, about 1965. In fact, local residents noted excessive sedimentation in the winters of 1966-68. Present cross-sectional surveys show degradation in the logged reach and aggradation near the mouth, suggesting the slug of sediment is being transported downstream. A high bedload rate in this reach is also indicated by field observa

tions. Channel length decreased, and the number and size of midchannel bars increased. A similar trend is seen 6.2 km upstream (fig. 2C). Road con

struction and logging in the late 1960’s caused aggradation of the adjacent stream channel. Presently, net degradation is occurring there, and channel widening and braiding occur downstream for 0.8 km. This accumulation constitutes a second distinct wave of sediment in Big Beef Creek, and it will affect the stream for several years to come.

### 107 

## CONCLUSION In forested drainage basins, sediment is supplied to stream channels by soil creep and mass movements. Channel form is adjusted to the amount of sediment supplied. Logging, road construction, and urban development remove vegeta

tion, disrupt the ground surface, and cause accelerated erosion from sheetwash, mass movements, and rainsplash erosion, and hence increase the sediment load. Under undis

turbed conditions, Big Beef Creek received 500 t of sedi

ment per year. Land-use changes increased the load to 4200 t/year. Construction of a weir at the mouth of the stream caused the bedload fraction to be caught above the weir, where an average of 1400 t of sand and gravel are deposited annually. Stream-channel dimensions changed in response to in

creased sediment load. The channel in 1978 was wider and shallower than the 1970 channel, more gravel bars were present, and sinuosity decreased slightly in areas of high sediment transport. Mean flow velocity remained about constant. The changes result in an increase in shear stress along bed and banks, which in turn results in a higher rate of sediment transport. Before disturbance, sediment-trans

port rate of bedload was probably 300-600 t/year; at present, it is 600-1400 t/year. Channel adaptations are restricted by hydraulic constraints described in the continuity and Manning equations. Velocity and slope are relatively conservative parameters, and most change is accommodated by width and depth. The stream channel presently has about 1 000 000 t of sediment in storage. Active sediment, 49 000 t, moves an average of 200 m/year in the main channel. Thus sediment placed in the channel by present disturbances will take on the order of 20 to 40 years to be removed.



## LITERATURE CITED Anderson, H. W. 1954. Suspended sediment discharge as related to streamflow, topography, soil, and land use. Trans. Am. Geophys. Union 35(2):268-281. Barnes, H. H., Jr. 1967. Roughness characteristics of natural channels. U.S. Geol. Surv. Water Supply Pap. 1849, 213 p. Cederholm, C. J. 1972. The short-term physical and biological effects of stream channelization at Big Leaf Creek, Kitsap County, Washington. M. S. thesis. Univ. Wash., Seattle. 80 p. Dietrich, W. E. 1975. Sediment production in a mountainous basaltic terrain in central coastal Oregon. M. S. thesis. Univ. Wash., Seattle. 81 p. Diseker, E. G., and E. C. Richardson. 1961. Roadside sediment production and control. Am. Soc. Agric. Eng. 4:62-68. Einstein, H. A. 1950. The bedload function for sediment transportation in open channel flows. U.S. Dep. Agric. Tech. Bull. 1026, 71 p. Emmett, W. W. 1976 . Bedload transport in 2 large gravelbed rivers, Idaho and Washington. _In Proc. 3d Fed. Interagency Sediment. Conf” Water Resourc. Counc., p. 4-101-4-114. Denver, Colo. Everett, V. R. 1963. Slope movement, Neotoma Valley, southern Ohio. Rep. Inst. Polar Stud. No. 6, 16 p. Ohio State Univ., Columbus. Fredriksen, R. L. 1970. Erosion and sedimentation following road construction and timber harvest on unstable soils in three small western Oregon watersheds. USDA For. Serv. Res. Pap. PNW-104, 15 p. Pac. Northwest For. and Range Exp. Stn., Portland, Oreg. Graf, W. H. 1971. Hydraulics of sediment transport. 513 p. McGraw Hill, New York. Hollingshead, H. 1971. Sediment transport measurements in gravel rivers. J. Hydraul. Div., Am. Soc. Civ. Eng. 97(HY11):1817-1834. Hornbeck, J. W., and K. G. Reinhart. 1964. Water quality and soil erosion as affected by logging in steep terrain. J. Soil Water Conserv. 19(1):23-27. Kirkby, J. J. 1967. Measurement and theory of soil creep. J. Geol. 75:359-378. Kuenen, P. H. 1956. Experimental abrasion of pebbles. II. Rolling by current. J. Geol. 64:336-368. Leopold, L. B., and T. Maddock. 1953. The hydraulic geometry of stream channels and some physiographic implications. U.S. Geol. Surv. Prof. Pap. 252, 57 p. Washington, D.C. Leopold, L. B., M. G. Wolman, and J. P. Miller. 1964. Fluvial processes in geomorphology. 522 p. W. H. Freeman, San Francisco. Mackin, J. H. 1948. Concept of the graded river. Geol. Soc. Am. Bull. 59:463-512. Madej, M. A. 1978. Response of a stream channel to an increase in sediment load. M. S. thesis. Univ. Wash., Seattle. 111 p. Meyer-Peter, E., and R. Killer. 1948. Formulas for bedload transport. Proc. 2d Int. Assoc. Hydr. Struct. Re s., p. 39-64. Stockholm. Milhous, R. T. 1975. Sediment transport in a gravelbottomed stream. Ph. D. thesis. Oreg. State Univ., Corvallis. 232 p. Pickup, G. 1976. Adjustment of stream channel shape to hydrologic regime. J. Hydrol. 30:365-373.