Response and recovery of a subalpine stream following a catastrophic flood
JOHN PITLICK Department of Geography, University of Colorado, Boulder, Colorado 80309
ABSTRACT km downstream of the terminus of the depo- clear why some floods are more effective in sitional zone ranged from 5 x 106 to 11 x 106 modifying stream channels than others. Wol- The July 15, 1982, Lawn Lake flood in kg/yr, but showed no significant decline over man and Gerson (1978) suggested that geo- Rocky Mountain National Park, Colorado, the 5-yr study period. The average rate of morphic effectiveness was determined largely was caused by the failure of a 79-yr-oId earthen bed-load transport through this reach was at by climate, but topography, lithology, and dam. Peak discharges of the flood far exceeded least 100 times greater than before the Lawn vegetation are important as well. They further naturally occurring flows, and it caused severe Lake flood, but few discernible channel suggested that geomorphic effectiveness channel disturbance along most of Roaring changes resulted from the higher loads. could be evaluated in terms of the length of River and some parts of Fall River. This study time required for a landform to recover its documents the geomorphic response of a 5-km INTRODUCTION prior condition. What constitutes "recoveiy" reach of Fall River in the 5 yr following the in the case of rivers remains somewhat am- flood. In 1983, the first year after the Lawn Considerable effort has been spent trying to biguous, but examples might include the Lake flood, snowmelt flows were well above predict how alluvial rivers will respond to reconstruction of floodplains (Hack and average. These high flows together with very sudden changes in discharge (Q) and sedi- Goodlett, 1960; Schumm and Lichty, 1963) or
high sediment yields from Roaring River re- ment load (¡2S). These changes may result the restoration of preflood channel width (Os- sulted in significant geomorphic changes on from disturbances within a watershed caused, terkamp and Costa, 1987), hydraulic geome- reaches of Fall River downstream. During the for example, by floods, landslides, defores- try (Lisle, 1982), bed elevation (Kelsey, 1983 snowmelt runoff, -15.5 x 10* kg of bed- tation, or dams. When the normal water- and 1980), and sediment loads (Newson, 1980). load sediment was eroded from the upper part sediment-discharge regimens of a river are When viewed in this context, recoveiy es- of the study area. These loads were at least altered, two questions immediately arise: (1) sentially involves the re-establishment of a 1,000 times higher than before the Lawn Lake how will the channel adjust its morphology quasi-equilibrium channel in response to flood. Most of this sediment was then deposited and (2) how rapidly will these adjustments changes in discharge and sediment load. in a highly sinuous reach of Fall River in the take place? To answer these questions, we In this paper, I describe how two streams lower part of the study area. This reach had not could take the approach of using a physically in Rocky Mountain National Park recovered been much aifected by the Lawn Lake flood, based model of channel evolution, but to do from a dam-break flood. The flood occurred but sedimentation during the period of high this, we would need rather specific informa- on July 15, 1982, when an earthen dam on flow in 1983 completely filled in the channel, tion about Q or Qs and how they change with Lawn Lake failed, unleashing a torrent to the resulting in the formation of a continuous 2.3- time. Our ability to model or measure these valleys of Roaring River and Fall River km-long depositional zone. In 1984, sediment processes at watershed scales is still very lim- (Fig. 1). Prior to this flood, Roaring River and yield from Roaring River declined dramati- ited, so for the present, we must rely on direct Fall River were typical of alpine-subalpine cally, and this trend continued for the next 3 yr. observation and empiricism to understand streams in the Colorado Front Range: they By 1987, the bed-load sediment yield in the how channels change in response to sudden had stable channels and carried low sediment upper reaches of Fall River was only about disturbances. loads (Caine, 1986). The Lawn Lake flood 0.4 x 10'' kg/yr. The decline in sediment loads Of various watershed-scale disturbances, radically altered this scenario by causing ex- resulted in progressive erosion and recovery of floods have received perhaps the most atten- tensive erosion along the entire length of the original channel of Fall River in the depo- tion from geomorphologists. Indeed, the lit- Roaring River from Lawn Lake to its conflu- sitional zone reach. Recoveiy in the upstream erature contains many examples and descrip- ence with Fall River (Fig. 1). Where the two part of the depositional zone was complete by tions of how floods have modified stream rivers join, the flood deposited a large alluvial 1985. Recovery in the downstream part of the channels (see Baker and others, 1988, or fan, burying — 1 km of Fall River. The flood depositional zone took longer because of the Beven and Carling, 1989). From this work, it continued at much lower velocity through a continued supply of sediment and because the seems clear that the largest floods (meaning low-gradient valley known as Horseshoe sediment was mobile less of the time. As of "large" in terms of discharge per unit drain- Park (Fig. 1). This 4-km reach, which owes its 1987, about 80% of the material initially stored age area or in relation to the mean annual name to the tortuous meanders of Fall River, in the sedimentation zone had been eroded. flood) occur in small drainage basins in arid was left virtually unchanged because the Bed-load sediment yields at a sampling site 1 and semiarid regions (Costa, 1987). It is less flood was dispersed over much of the valley
Data Repository item 9316 contains additional material related to this article.
Geological Society of America Bulletin, v. 105, p. 657-670, 13 figs., 2 tables, May 1993.
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of which falls as snow between the months of "^105 °40' October and May. The bulk of runoff occurs Study m in the 3-mo period of spring and summer \ _ s • Area snowmelt from May through July. It is during Denver• this high-flow period that most sediment \ transport and channel change normally oc- Mummy COLORADO curs. A point to emphasize here is that, in Range _ ^ ^ alpine basins of the Colorado Front Range, peak discharges generated by snowmelt are ROCKY N not "large" in terms of discharge per unit MOUNTAIN A drainage area (Jarrett, 1990) or in terms of the mean annual flood (Pitlick, 1988). Flood-fre- NATIONAL quency curves for streams in this area have PARK pronounced negative skew, and they become asymptotic above a return period of —100 yr. This suggests an upper limit to the magnitude of snowmelt-generated floods in alpine-sub- alpine basins of Colorado, which is simply the result of there being a limit to the amount of energy available in the diurnal snowmelt cy- cle. Typically, the discharge of a 100-yr flood on an alpine stream in this area is not even twice the discharge of the 2-yr flood (Table 1). The effect of this hydrologic regimen is that Fall River experiences a relatively narrow Estes range of peak discharges, although in some Park years, moderate flows persist for several weeks. Roaring River and Fall River both flow within broad, glaciated valleys, and their Figure 1. Map of Rocky Mountain National Park and vicinity. channels are largely detached from hillslopes. Roaring River flows from Lawn Lake down a hanging valley with an average gradient of about 0.10. Above its confluence with Fall River, Roaring River has a drainage area of floor and its power was greatly reduced. At summarize the results of this work, with a 33 km2 (Fig. 1). Fall River drains similar ter- the downstream end of Horseshoe Park there specific focus on processes of channel adjust- rain, but flows down the valley carved by the are a series of late Pleistocene terminal mo- ment in a mountain stream subject to varying main glacier in the area. The average gradient raines beyond which the gradient of Fall River sediment supply. The Lawn Lake flood was of Fall River in Horseshoe Park is 0.003. increases. Here, the flood again gathered not spawned by a natural hydrologic event, of Through most of this area, Fall River flows in power, causing intermittent erosion and dep- course, but the sudden disturbance of other- a highly sinuous channel (average sinuosity in osition for the next 8 km. The flood swept wise stable stream systems, for example, due lower reaches is 2.5) cut in what appear to be through the town of Estes Park and was fi- to landslides or forest fires, is not so unusual glacial lake sediments. The drainage area of nally stopped when it reached Lake Estes in mountain areas. It is in this context that the Fall River where it exits Horseshoe Park (Fig. 1). The flood caused $31 million in dam- Lawn Lake flood and subsequent channel re- (Fig. 1) is 90 km2. age, and three lives were lost (Jarrett and covery are of broader interest. Details of the Lawn Lake flood are de- Costa, 1986). scribed thoroughly by Jarrett and Costa When I first visited this area in March 1983, STUDY AREA (1986) and Blair (1987). A peak discharge es- it was clear that the ensuing spring snowmelt timated by the slope-area method of about 500 would cause erosion on the Roaring River Fall River and its major tributary, Roaring m3/s occurred just downstream of Lawn fan, and this would greatly increase the sed- River, drain mountainous terrain within Lake. Above the Roaring River alluvial fan, iment load of Fall River. Presumably, a new Rocky Mountain National Park (Fig. 1). The the peak discharge of the flood was estimated course would be established across the fan, area is underlain by Precambrian crystalline to be 340 m3/s; Jarrett and Costa (1986) sug- and the meandering reaches below would rocks, which are mantled in many places by gested this was about 30 times the 500-yr flood have to adjust to the increased sediment load. glacial deposits. Soils are thin and weakly de- for Roaring River. The peak discharge on Fall Thus, I began a 5-yr field study to monitor veloped. A forest cover is found up to an River near the outlet of Horseshoe Park changes in the morphology and sediment load elevation of —3,500 m, above which tundra (Fig. 1) was estimated to be 200 m3/s, which of Fall River. The accessibility of Horseshoe and bare rock are common. Average annual is about eight times the 500-yr flood for this Park allowed me to document these changes precipitation in the area varies between 500 stream (Table 1). Two aspects of the Lawn in some detail. The purpose of this paper is to and 1,000 mm (Jarrett and Costa, 1986), most Lake flood are important in the context of this
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TABLE 1. ESTIMATES OF PEAK DISCHARGE ON FALL RIVER AND UNREGULATED STREAMS NEARBY 1975 by M. P. Mosley (1983, written com- mun.) clearly indicated that, except for the Station Fall River at Big Thompson River S. St. Vrain Creek Middle Boulder Creek Estes Park* at Estes Park near Ward at Nederland area covered by the alluvial fan, the reach of Fall River in Horseshoe Park was not signif- (USGS no.) (6732500) (6733000) (6722500) (6725500) icantly modified by the Lawn Lake flood. 2 Drainage area (km ) 103 355 37 94 The characteristics of the Roaring River Years of record 22 44 24 77 alluvial fan, as described by Jarrett and Costa (1986) and Blair (1987), are as follows: the fan 3 Return period (yr) Discharge (m /s) covers an area of 0.25 km2, and it has a max- imum thickness of 14 m and a volume of about l.U 3.9 17.6 4.2 8.1 3 1.50 6.3 26.3 6.2 11.7 280,000 m . Sediment in the fan was derived 2.0 3.0 30.0 7.0 13.0 primarily from a lateral moraine immediately 5.0 12.0 38.9 8.9 16.4 10 14.5 43.4 9.9 18.0 upstream that contains 60% boulders, 13% 25 17.4 47.9 10.8 19.6 50 19.5 50.6 11.4 20.6 cobbles, 14% pebbles, 3% gravel, 10% sand, 100 21.4 52.9 11.8 21.4 and <1% silt and clay. The fan was deposited 200 23.2 54.8 12.2 22.0 500 25.5 56.9 12.6 22.7 in three phases, with each being represented by a distinctive lobe. The proximal and me- * Nan recording crest gauge. dial lobes contain the coarsest debris. The distal lobe contains sediment grading from cobbles at the apex to medium sand at the study. First, the flood caused extensive ero- next year's snowmelt. Second, prior to the terminus. The abundance of sand in the distal sion and sedimentation along Roaring River flood, Fall River flowed within a highly sin- lobe of the alluvial fan was particularly sig- (Fig. 2a). In steeper reaches, the flood cut a uous channel bounded by well-vegetated nificant because, under normal snowmelt channel as much as 15 m deep, and the un- banks and a cobble bed. The channel was thus flows, Roaring River and Fall River could stable cut banks along the channel began col- quite resistant to erosion. Using data from carry this size sediment rapidly to the un- lapsing immediately after the flood passed. In cross sections surveyed in Horseshoe Park disturbed reaches. The Lawn Lake flood lower gradient reaches, the flood deposited a by Jarrett and Costa (1986) and my own field also left a network of distributary channels series of discontinuous terraces and alluvial surveys, I estimate that the shear stress in the on the fan, most of which were small in fans, the most spectacular of which was de- channel of Fall River during the Lawn Lake comparison to the reaches of Fall River posited at the mouth of Roaring River flood was less than two times the normal that were unchanged by the flood. It was (Fig. 2b). Thus, erosion caused by the flood bankfull shear stress and probably not more clear, therefore, that with the onset of the made available a tremendous amount of loose than three times the critical shear stress, that 1983 snowmelt, these channels would en- sediment. Because the flood occurred in mid- is, the stress needed to initiate bed-material large and become unstable. It was not July, however, well after the peak of the 1982 movement. Comparison of postflood obser- clear, however, what effect this instability snowmelt, most of this sediment remained at vations with ground photographs and bed ma- would have on the meandering reach of the base of the eroded cliff faces awaiting the terial samples taken in Horseshoe Park in Fall River downstream.
Figure 2. (a) Eroded channel of Roaring River, looking upstream. Channel is about 6 m wide, and cut bank in left of photo is about 5 m high, (b) Roaring River alluvial fan. Roaring River enters from the left and joins Fall River just downstream from the outlet of the lake. The lake formed when the drainage of Fall River was partially blocked by the alluvial fan.
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Figure 3. Map of Horseshoe Park showing the locations of selected cross sections (XS), sediment sampling stations (FR), and other features referred to in the text.
METHODS reach where Fall River exits Horseshoe Park and grain-size distribution for each bed-load (Fig. 3). At this point, the channel has a bank- measurement.1 In March 1983,1 began setting a network of full width of 7 m and a slope of 0.0015. This Finally, a water-stage recorder and staff 80 channel cross sections on Fall River be- section remained stable except for minor fluc- gauge were installed on the U.S. Highway 34 tween the mouth of Roaring River and the tuations in bed elevation due to dunes. Flow bridge where it crosses Fall River (Fig. 3). downstream end of Horseshoe Park (Fig. 3). and sediment discharge measurements were Current-meter measurements were made of- These cross sections were surveyed at least made at these sites almost daily during the ten to establish discharge-gauge height rating once each year, and many were surveyed sev- annual 4- to 6-wk period of snowmelt runoff. curves for both sediment sampling sites; these eral times per year. Point-count samples of The majority of sediment-discharge meas- rating curves were adjusted periodically. In- the bed material were repeated at selected urements include samples of only bed load stantaneous peak discharges and mean daily cross sections. and not suspended load. The emphasis here discharges of Fall River in Horseshoe Park Next, I established stream-gauging and was on samplingbed load because, after 1983, were determined from the strip-chart record sediment-sampling stations. The upstream suspended sediment concentrations in Fall and later compared with data from a crest station, FR-1, was established in late May River were typically < 100 mg/1. Bed load was gauge on Fall River near Estes Park (USGS 1983 in a straight reach about 400 m down- measured with a hand-held Helley-Smith station number 673250) and a continuously stream of the alluvial fan (Fig. 3). At this lo- sampler having a 7.6-cm nozzle. A bed-load recording gauge on the Big Thompson River, cality, Fall River flows within a well-vege- sample includes all of the bed load measured to which Fall River is a tributary (USGS sta- tated channel having a bankfull width of 8.5 m at 14 to 18 points spaced equally across the tion number 673300). I considered the latter and a slope of 0.0032. The channel here was channel. All bed-load samples were dried, record the more useful one because it was stable for the entire study period, except in weighed, and sieved. In all, more than 500 June 1983, when bed-load transport rates bed-load samples were taken in Horseshoe Park. Included as part of this data set are the were high and dunes were migrating over the 'GSA Data Repository item 9316, four tables, is bed. The downstream station, FR-2, was es- location, date, time, discharge, mean flow ve- available on request from Documents Secretary, tablished in July 1983 in the very sinuous locity, flow depth, bed-load transport rate, GSA, P.O. Box 9140, Boulder, CO 80301.
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longer (Table 1) and because, in addition to flood. High flows in spring of 1983, together the reach slope (5) was 0.0068, and the median peak flows, it includes a continuous record of with the tremendous influx of sediment from grain size (Dso) of the bed material was 16 daily flows. On the basis of these compari- Roaring River, immediately caused channels mm. By June 1985, the channel had widened sons, I found that the discharge of Fall River on the alluvial fan to widen and become un- to 11 m (Fig. 5), andZ)50 had coarsened to 32 in Horseshoe Park was consistently 29% of stable. Distributary channels on the fan fre- mm, while the slope did not change. The crit- the discharge of the Big Thompson River in quently avulsed, and new courses were carved ical shear stress (and hence critical flow Estes Park. Not coincidentally, the drainage across the surface. The unconsolidated finer- depth, dc) for 32-mm bed material particles area of Fall River in Horseshoe Park is 29% grained flood deposits were continually re- can be estimated using the Shields parameter, of the drainage area of the Big Thompson worked as a result of these avulsions, and the River in Estes Park. I thus adopted the con- sand-size fraction of the fan sediment was vention of taking 29% of the discharge of the transported rapidly downstream. Big Thompson River in Estes Park to get the Sediment yields from Roaring River de-
discharge of Fall River in Horseshoe Park for clined significantly in 1984. By the end of the where 7 and ys are the specific weight of water the period prior to my study. From this proxy 1984 snowmelt, most of the steep gulley walls and sediment, respectively. Taking T* = 0.06
flow record, I then constructed flood-fre- along Roaring River had stabilized, and much (the appropriate value for Dso) and rearrang- quency and flow-duration curves for Fall of the loose sediment near the channel had ing, the solution of equation 1 indicates that
River in Horseshoe Park, which allowed me been removed. In addition, nearly all of the Ds0 begins to move at dc = 0.5 m. This flow to place the flows observed from 1983 to 1987 surface of the distal alluvial-fan lobe had been depth corresponds exactly to the observed in a long-term context. reworked, leaving a lag of gravel-size sedi- bankfull stage (Fig. 5). At this stage, the dis- ment. The channel of Fall River within the charge is 9 m3/s, which is equivalent to the alluvial-fan reach remained stable from this mean annual flood. Thus, within 2 yr of the RESULTS time on. Lawn Lake flood, Fall River had formed a It would be overstating the case to say that channel wherein the bed material just begins Hydrology in the alluvial-fan reach Fall River has "re- to move at the bankfull discharge. covered, '' because the newly formed channel Snowmelt discharges on Fall River ranged is vety different from the preflood channel. Changes in Bed Material from well above average in 1983 to well below From 1984 on, however, channel changes in average in 1987 (Fig. 4). Peak and mean daily this reach were minor. Sediment supply from The sequence of channel evolution de- discharges in 1983, the year immediately after Roaring River remained low, but it did not scribed above was accompanied by distinct the Lawn Lake flood, were higher than in any cease altogether, and Fall River continued to changes in the bed material and sediment load other year of this study (Fig. 4). The peak transport small but measurable amounts of of Fall River. Prior to the Lawn Lake flood, discharge in 1983 of 11.9 m3/s was equivalent bed load out of the alluvial reach. This con- the bed material of Fall River was gravel- to a 5-yr flood (Table 1). Perhaps as important dition, where the channel is stable yet capable sized, but the flood left deposits in the distal was the fact that mean daily discharges of 7 of transporting the load supplied from up- lobe of the alluvial fan (cross sections 17 and m3/s (~1.5-yr flood) were exceeded for more stream, could be interpreted as a state of equi- 18, Fig. 2) that were much finer. This sedi- than 3 wk (Fig. 4). The volume of runoff pro- librium. Indeed, many self-formed gravel-bed ment was easily moved, and not surprisingly, duced from June 14 to July 13,1983, ranks as rivers appear to have mobile beds but stable the bed load sampled at FR-1 at the peak of the highest 30-d mean discharge in 39 yr of banks (Andrews, 1984). Parker (1979) pointed the 1983 snowmelt (6-13-83) had nearly the record. Runoff throughout the region was out the paradox of this condition, but he re- same grain size distribution as the surface de- high in 1983 because of heavy winter snow- solved it by showing that the distribution of posits on the alluvial fan (Fig. 6a). The bed falls associated with an El Nino event. The shear stress across the bed of a rectangular material coarsened over the next 2 yr to form years 1984 through 1986 were more or less channel is not uniform. Parker (1979) found a gravel armor (Fig. 6a), which could be average in terms of peak discharges and total that the local boundary shear stress, as given moved by flows that occur several days per runoff volumes. In 1987, the peak discharge by the depth-slope product, would always be year. At these flows, movement of particles is and total runoff were somewhat below aver- greater in the center of the channel than near probably sporadic and the armor remains age (Fig. 4). Fall River, like other streams the banks. As a result, at bankfull flow, the largely intact, protecting the bulk of sediment draining alpine basins in the Front Range, is shear stress (T) in the center of the channel can underneath from being eroded. Still, some of not likely to experience flows much greater be above the threshold for motion (Tc) and the sediment below is eroded and transported than those observed in this 5-yr period. As sediment transport can occur, while near the as bed load. The bed load sampled at FR-1 in will be shown, the process of geomorphic re- banks, T < Tc, no sediment transport occurs, June 1985 had virtually the same size distri- covery on Fall River appears to become less and the banks remain stable. The river thus bution as the subsurface bed material sam- sensitive with time to variations in the hydro- adjusts its width (and its depth) to the point pled on the alluvial fan in July 1985 (Fig. 6b). logic regimen. where it is just able to transport the supplied This again implies that, as early as 1985, Fall load at the bankfull discharge. River was reaching a near-equilibrium state, Channel Changes in Upper Horseshoe Park The channel of Fall River on the alluvial fan because if the subsurface bed material was is entirely self-formed in sediment deposited the primary source of bed load at FR-1, and The pronounced channel changes that oc- by the Lawn Lake flood, and thus cross sec- the channel was not degrading, then there curred within Horseshoe Park owe their his- tions surveyed in this reach offer a test of must be a balance between the load supplied tory in part to the particular sequencing of Parker's theory (Fig. 5). At the time of the first from upstream and the load transported off runoff events that followed the Lawn Lake survey (May 1983), the channel was 7 m wide, the fan.
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MAY JUNE JULY AUGUST MAY JUNE JULY I AUGUST MAY JUNE JULY I AUGUST
12 I I II I I I II I! I I II I I I I I I I I I I I I II II I I I I I I I I I II I I I I II 1986 ^ggy 5-year flood wm
10
2-year flood
^ 8 o Figure 4. Mean daily discharges on Fall River for periods between May 1 and August 31 from 1983 O®) 6 through 1987. The single dark circle marks the instantaneous peak dis- u CO charge for the year. The horizontal bars correspond to discharges of the 5- and 2-yr floods.
MAY I JUNE JULY AUGUST MAY I JUNE I JULY AUGUST
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2604.5 bed load per day, which is nearly three orders of magnitude lower than the rates measured in 1983. Superimposed on the log-linear decline in bed-load transport are annual fluctuations corresponding to the rise and fall of the snow- 2604.0 melt hydrograph. A time series of average boundary shear stress at FR-1 shows a slight decreasing trend (Fig. 7b), but this trend is not commensurate with the overall decline in
Ul 2603.5 bed-load transport rates. On the basis of these data, it appears that the long-term decline in bed-load transport in the upper reaches of Fall River was governed mostly by sediment supply rather than by sediment transport 2603.0 capacity. 30 35 45 50 55 A similar time series of median grain size
Distance Left Endpoint (m) (Dso) shows a slight coarsening of the bed load (Fig. 7c). At the start of the 1983 snowmelt, Figure 5. Cross section of Fall River in the upstream part of the alluvial fan showing changes the bed load at FR-1 was medium sand, de- in morphology at three different times. The flow level indicated is that required to move the median rived primarily from the distal alluvial fan. grain size of the bed material as it existed in June 1987. Subsequently, the bed load at FR-1 coars- ened, but still much of it was sand-sized. As discussed above, sediment stored beneath the Sediment Transport, FR-1 June 1983 (Fig. 7a) when flows were highest armor layer in the alluvial-fan reach was the and erosion was occurring upstream. During source for this bed load. The quasi-steady 3 Tlie description of postflood geomorphic this period, as much as 750 Mg (Mg = 10 kg) trend in Dsu of the bed load (Fig. 7c) suggests response of Fall River in upper Horseshoe of bed load was transported per day at FR-1. that there was continued movement of sedi- Park concludes here with a discussion of From mid-June 1983 on, bed-load transport ment in the armor layer, but as the declining trends in bed-load transport at the FR-1 sam- rates declined steadily (Fig. 7a). The overall trend in bed-load transport rates (Fig. 7a) in- pling station. When plotted as a semicontin- trend of this decline is log-linear (note that the dicates, decreasingly less of the bed was uous time series, these data provide a remark- ordinate scale is logarithmic), implying an ex- mobile. ably consistent picture of Fall River's ponential decay in bed-load transport. By late Annual sediment yields of Fall River in up- response to the Lawn Lake flood (Fig. 7). June 1987, when the last measurements were per Horseshoe Park were estimated by inte- Bed-load transport rates reached their peak in made, Fall River was carrying only 3 Mg of grating the area under the bed-load time-se-
100 surface, 7-2-85 •o- subsurface, 7-2-85 80 - -0 - bed load, 6-7-85
g 60 II
SI 40
20
1 10 0.1 1 10 100 Grain Size (mm) Grain Size (mm)
Figure 6. Grain-size distributions of bed material in the distal reach of the alluvial fan and bed load at FR-1. (a) Comparison of bed material prior to the Lavm Lake flood with a sample obtained at cross section 17 at the start of the 1983 snowmelt, and bed load at FR-1 at the peak of snowmelt. The preflood grain-size distribution is based on samples taken in Horseshoe Park in 1975 by M. P. Mosley (1983, written commun.). (b) Comparison of surface and subsurface bed material sampled at cross section 18 in 1985, and bed load at FR-1.
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Upstream Sediment Sampling Station, FR-1 the alluvial fan the equivalent of 28,000 yr of 10° bed load. These comparisons are of little sig- v> 1 nificance, but they illustrate how it takes a D> truly extraordinary event like the Lawn Lake flood to push an alpine fluvial system much o beyond its normally very stable condition. 1 = 10' O «O o r o û « ° ° G a. 0 o âo Channel Changes in Lower Horseshoe Park » % ^ O °o% O O © o o8® ° A complex pattern of deposition and ero- o»« P„ o o § 10'2 - - . . o 0 0o sion occurred in the reaches of Fall River «"•"-••-.A downstream of the U.S. Highway 34 bridge. ° Co«4 a oO COD Recall that the Lawn Lake flood did not sig- O Nonetheless, the 1983 bed-load sediment the 5 yr after the Lawn Lake flood than it did 3 Note: values are in megagrams (10 kg) per year. yield exceeded the combined total of all sub- in the previous 350 yr. Extending this reason- *Too few measurements made to accurately determine sedi- sequent years. Over 5 yr, —26.4 x 103 Mg of ing further, the Lawn Lake flood deposited in ment yield. 664 Geological Society of America Bulletin, May 1993 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/5/657/3381740/i0016-7606-105-5-657.pdf by guest on 29 September 2021 meters Figure 8. September 1983 aerial photographs of Fall River: (a) the downstream end of the depositional zone showing selected cross sections; (b) the reach downstream of the depositional zone showing selected cross sections and the FR-2 sediment sampling station. Geological Society of America Bulletin, May 1993 665 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/5/657/3381740/i0016-7606-105-5-657.pdf by guest on 29 September 2021 J. PITLICK 2591 Cross Section 54 -g- 2590 ff" as in 2589 pre-1983 6-20-84 5-26-86 6-24-87 10 15 20 5 10 15 Distance From Left Endpoint (m) Distance From Left Endpoint (m) Figure 9. Channel cross sections in the (a) upstream and (b) middle parts of the depositional zone. as before the Lawn Lake flood (Fig. 9a). By sition had been ubiquitous here as well. The in this reach in July 1983 was 80% sand and July 20,1983, the channel had aggraded to the 1984 survey of cross section 54 shows Fall 20% gravel, almost identical to the bed load level of the floodplain (Fig. 9a). Subsequent River in its aggraded state (Fig. 9b). By 1986, measured at FR-1 in June 1983 (Fig. 10a). measurements at this site revealed that aggra- the channel had begun to recover its original Calculation of the critical shear stress using dation was short lived. A survey in mid-July configuration, but some aggradation was still Equation 1 and specific values for this reach 1984 (Fig. 9a) showed that most of the sedi- evident in the survey of June 1987 (Fig. 9b). of s = 0.0035 and£)50 = 2.0 mm indicates that ment in the channel had been eroded, and Fall Thus, in contrast to the upstream reaches, the sediment at cross section 46 could be moved River had nearly recovered its pre-1983 mor- downstream reaches of Fall River had not by a flow 6 cm deep. A discharge of this sort phology. From then on, channel changes in fully recovered after 5 yr from the high sed- would be exceeded about 50% of the time. By this reach were minor. iment loads of 1983. July 1984, most of the sand was removed, and Cross sections in the downstream end of Recovery was most rapid in the upstream a gravel armor with a size distribution ap- the depositional zone were not surveyed until part of the depositional zone because of the proaching that of the pre-1983 bed material 1984, but it was clear from looking at the ex- drop in sediment loads from the alluvial-fan had formed (Fig. 10a). Armor layers were tent of overbank deposits in the field and on reach, and because sediment in the channel slower to form in the lower part of the dep- 1983 aerial photographs (Fig. 8a) that depo- could be easily transported. The bed material ositional zone. Once in place, the armor lay- 100 l| , ,11. ,T,r 100 T-e- pre-flood -•- pre-flood A' -o- bed load, 6-20-83 -o- 7-20-83 --0-- ••A» 9-3-85 80 7-20-83 80 -A- 7-3-84 -«-- 5-22-87 5-22-87 g 60 e 60 I*T* il c «c o: a I : SI 40 ® 40 / : a // -4 20 20 / X / M--0" p-i-H-tr* 0.1 1 10 0.1 1 10 100 Grain Size (mm) Grain Size (mm) Figure 10. Grain-size distributions of bed material in the depositional zone, (a) Comparison of bed material prior to the Lawn Lake flood (M. P. Mosley, 1983, written commun.), bed load at FR-1 at the peak of the 1983 snowmelt, and bed material at cross section 43 in July 1983, July 1984, and May 1987. (b) Comparison of bed material prior to the Lawn Lake flood, and surface bed material at cross section 54 in July 1983, September 1985, and May 1987. 666 Geological Society of America Bulletin, May 1993 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/5/657/3381740/i0016-7606-105-5-657.pdf by guest on 29 September 2021 SUBALPINE STREAM FOLLOWING CATASTROPHIC FLOOD Distance From Left Endpoint (m) Distance From Left Endpoint (m) Figure 11. Channel cross sections in the reach immediately downstream of the depositional zone. ers slowed erosion considerably, but not to shown. Figure 12 shows that by July 1983, the a mass of about 37.5 x 103 Mg, which is more the point where it ceased altogether. The bed entire length of channel from cross section 40 than twice the bed load measured at FR-1 in material at cross section 54 on May 22,1987, to 57 had aggraded by about 1.0 m (Fig. 12). 1983(15.5 x 103Mg). Assuming my estimates was predominantly gravel (Fig. 10b), which The "nose" of the depositional zone is clearly of volume and bulk density are not far off, the could be moved under moderate flows. Cal- seen between cross sections 57 and 60; in this discrepancy in sediment yield either reflects culation of the critical shear stress using area, about 0.5 m of aggradation is evident. the fact that some of the sediment in the de- Equation 1 and specific values of 5 = 0.0025 From cross section 60 to FR-1, there was only positional zone was transported as suspended and Dso = 10 mm, indicates the bed material —0.3 m of aggradation, and there was little load, which was not sampled systematically at at cross section 54 would move at a depth of loss of channel capacity (Fig. 12). By the end FR-1, or that the 1983 series of bed-load mea- 0.4 m or a discharge corresponding to 4.5 of the 1983 snowmelt, about 25,000 m3 of sed- surements at FR-1 is incomplete. m3/s. A flow of this magnitude would be ex- iment was stored in lower Horseshoe Park, By 1984, much sediment had been eroded ceeded about 18 d/yr, which is often enough most of it in the reach between the highway from the upstream end of the depositional to cause persistent erosion. bridge and the cut-off meander. At a bulk zone, but aggradation is still evident in the The enigma of the depositional zone was density of 1.5 Mg/m3, this volume equates to downstream reaches (Fig. 12). By 1987, only that it terminated so abruptly and that aggra- dation was never very significant in highly —I—i—I—I—|—i- 1 sinuous reach immediately downstream 1.5 r (Fig. 8b). After the experience of the 1983 ®2 section 46 54 57 60 67 FR-2 snowmelt, I expected that in 1984 sediment £ I • • • I would move en masse into this reach. This 1.0 c proved not to be the case. Repeated surveys o of cross sections in this reach showed that £> there was a small amount of aggradation in £ ui 0.5 1984 (Fig. 11), but it was clear that sediment S did not move through as a wave. The same m conditions prevailed through 1987. Obvi- a> a ously, channel capacity in this reach was suf- 2 0.0 ficient to convey most of the supplied load, 03 June1983 which was modulated upstream by the back- c • July 1983 water effect of the cut-off meander. a> -0.5 • July 1984 a> To summarize these data, changes in bed c * June 1987 co elevation at cross sections from the upper end f • bankfull O of the depositional zone to FR-2 were calcu- -1.0 lated for different time periods (Fig. 12). 2000 2500 3000 3500 4000 4500 5000 Changes in bed elevation are with reference to a common datum defined by the average Distance (meters) bed elevation in June 1983; points above the Figure 12. Change in average bed elevation at cross sections in lower Horseshoe Park. The dashed line indicate aggradation, and points points shown are changes in bed elevation referenced to the average bed elevation in June 1983 below the line indicate degradation. Bankfull (dashed line). Open symbols are estimates of average bed elevation based on observations of elevations at each cross section are also maximum scour and fill at individual cross sections. For clarity, not all surveys are shown. Geological Society of America Bulletin, May 1993 667 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/5/657/3381740/i0016-7606-105-5-657.pdf by guest on 29 September 2021 J. PITLICK Downstream Sediment Sampling Station, FR-2 series illustrate how the processes of recov- ery may be temporally disjointed over rela- tively short distances. DISCUSSION Although the Lawn Lake flood was not a natural event, the results of this study provide the basis for a general discussion of floods and their impacts on streams. Questions of scale and transferability will undoubtedly come to mind, but there are both parallels and con- trasts between my study and other studies of the fluvial response to large floods. i. 20 go a Geomorphic Effects of Large Floods ° a w 10 ^ The amount of erosion and deposition caused directly by the Lawn Lake flood was strongly influenced by the local topography, most of which is a relic of the last glaciation. Steeper reaches of Roaring River were scoured to bedrock, gentler reaches were sites of deposition, and wide, low-gradient reaches of Fall River were affected little. Abrupt changes in gradient and valley width are common in mountain streams, regardless of whether or not the area has been glaciated, and this makes it difficult to generalize about the role of catastrophic floods in small drain- 1983 1984 1985 1986 1987 age basins. Baker and Costa (1987) showed Figure 13. Trends in (a) bed-load transport rate, (b) average boundary shear stress, and (c) that floods achieve the greatest power (the median grain size at the downstream sampling site, FR-2. The series for each year includes only product of shear stress and velocity) in basins 2 measurements from June 1 to July 30. of 10-50 km , but it does not necessarily follow that floods on small, high-gradient streams are geomorphically the most effec- tive. Small streams typically have coarse bed 3,000 m3 of sediment (-12% of the initial vol- -30 x 103 Mg of bed load past the FR-2 materials and banks that are difficult to erode. ume) remained stored within this reach, and gauge. This number exceeds the 5-yr total at If thresholds for bed or bank erosion are the profile of average bed elevations followed FR-1 by —15%, which isn't likely considering greatly exceeded, however, floods can have a relatively smooth but slightly over-steep- some sediment remains in storage between catastrophic impacts, as witnessed on Cim- ened trend (Fig. 12). the two sites. Again, the discrepancy is prob- arron River (Schumm and Lichty, 1963), Cof- ably in the estimate of bed-load sediment fee Creek (Stewart and LaMarche, 1967), Sediment Transport, FR-2 yield at FR-1 in 1983, which I suggested above Plum Creek (Osterkamp and Costa, 1987), was perhaps 100% low. Assuming a bulk den- and Roaring River (this study). On the other The cut-off meander that formed the down- sity of 1.5 Mg/m3, the 4-yr total of bed-load hand, if sediment transport thresholds are stream terminus of the depositional zone sediment yield at FR-2 is equivalent to a vol- only slightly exceeded, erosion and deposi- slowed sediment movement, but did not stop ume of20,000 m3, which is 80% of the volume tion by floods tend to be spotty, as observed it altogether. A time series of bed-load trans- initially stored in the depositional zone. From on streams in Connecticut (Wolman and port at the FR-2 gauge shows that sediment cross-section measurements, I estimated that Eiler, 1958), in Wales (Newson, 1980), and on transport in the sinuous reaches in lower 88% of the sediment had been removed from Fall River in Horseshoe Park (this study). In Horseshoe Park was relatively steady the depositional zone, so these two estimates a study of the geomorphic effects of historic 2 (Fig. 13a). Each year there are rises and falls of sediment yield are in close agreement with floods in basins of 250-2,500 km in the cen- in bed-load transport related to snowmelt, but each other. In any event, transport rates at tral Appalachians, Miller (1990) found that overall, the trends in unit bed-load transport FR-2 were never as high as they were at FR-1 stream power was a poor predictor of geo- rate (Fig. 13a), boundary shear stress in 1983, although moderate rates persisted for morphic change. Miller (1990) concluded that (Fig. 13b), and median grain size (Fig. 13c) are a longer time. When compared to data from local site characteristics, such as channel consistent from year to year. Integrating the the upstream site, the time series of bed-load alignment or valley width, have as much to do area under the bed-load transport curve, I measurements at FR-2 indicates little in the with morphologic change as does the dis- estimate that over 4 yr Fall River transported way of recovery. Together these two time charge or power of the flood. The observation 668 Geological Society of America Bulletin, May 1993 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/5/657/3381740/i0016-7606-105-5-657.pdf by guest on 29 September 2021 SUBALPINE STREAM FOLLOWING CATASTROPHIC FLOOD that the Lawn Lake flood had such different sion in erosion was observed in the deposi- yield from an undisturbed watershed. From geomorphic effects on Roaring River and Fall tional-zone reach. Downstream-progressive 1984 on, sediment supply from Roaring River River supports this conclusion. trends in erosion and sediment transport ap- decreased, and the channel of Fall River on pear to be common in rivers overloaded with the alluvial fan stabilized. The trend toward coarse sediment. Gilbert (1917) was the first rapid recoveiy in the upper part of the study Role of Sediment Supply and Hydrology to identify such trends, but similar patterns of area is represented also by the exponential sediment movement were observed on rivers decay in bed-load measurements at a site It is common for sediment loads to initially in northern California following a large flood downstream of the alluvial fan. By 1987, the increase as the result of disturbance caused in 1964 (Kelsey, 1980; Madej, 1992). I would sediment yield of Fall River was 0.4 x 103 by a flood, and then to decline as hillslopes not characterize sediment transport on Fall Mg/yr. This was only 3% of the amount of and channels become more stable. The rate of River as "wave-like," but my observations of sediment carried in 1983, but probably close recovery thus depends on the rate of change erosion and deposition are not unlike what to pre-flood levels. The implication is that in sediment supply. If the channel continues has occurred on braided rivers in New within 5 yr the upper reaches of Fall River to be overloaded with coarse sediment, wid- Zealand (Griffiths, 1979; Beschta, 1983) and had nearly recovered from the effects of a ening will persist (Schumm, 1969); if the chan- New Guinea (Pickup and others, 1983). flood that in every sense was catastrophic. nel is starved of sediment, narrowing will re- Another trend revealed by the data from Considering the scale of the initial disturb- sult (Williams and Wolman, 1984). The time FR-1 is one of an exponential decline in sed- ance, this was unexpected, but it can be ex- it takes for a new equilibrium to be established iment loads. Similar trends have been re- plained qualitatively by the inherent stability appears to be quite variable, but it clearly ported in studies of rill erosion on hillslopes of the Roaring River and Fall River depends on the rate that processes within the near Mount St. Helens (Collins and Dunne, watersheds. watershed continue to supply sediment, and 1986), bed-load sediment transport following In the reach of Fall River in lower Horse- on the types of flows that can move sediment. floods in Wales (Newson, 1980), and changes shoe Park, the indirect effects of the Lawn Both of these factors played an important in channel-stored sediment following timber Lake flood were far more significant than the role in Fall River's response to the Lawn harvesting in low-order basins (Pitlick, 1992). direct effects. The flood produced few chan- Lake flood. Fall River was quick to re-estab- That the upper reaches of Fall River returned nel changes in this reach, but during a month- lish a stable channel on the alluvial fan largely to near-background sediment loads within 5 long period of high flow and high sediment because sediment yield of Roaring River de- yr of the Lawn Lake flood is remarkable con- supply in 1983, the channel aggraded to the creased rapidly. Most of the sediment in the sidering the magnitude of the event. Contrast level of the floodplain. This produced a con- Roaring River valley is of glacial origin, and it this with the bed-load data from FR-2, which tinuous 2,300-m-long zone of deposition. could not be moved by normal snowmelt showed little in the way of a trend toward Aggradation in the upstream reaches of the flows. The inherent stability of this watershed recoveiy, and the continuity in time is lost depositional zone was short lived, and Fall perhaps more than anything dictated the pat- over a very short distance of channel. River quickly recovered its pre-existing chan- tern of recovery on Fall River. This suggests nel. Recoveiy of channel morphology in the that watershed factors, such as relief, geol- upstream reaches of the depositional zone ogy, vegetation, or land use, are as important CONCLUSIONS was rapid because of the reduction in sedi- in governing rates of recoveiy as climate be- ment yield from the alluvial-fan reach and cause these factors control sediment supply. The Lawn Lake flood attained peak dis- because the sediment in the channel could be Hydrology still plays an important role in charges far outside the range of naturally oc- moved frequently. Recovery in the down- the recovery process. The snowmelt runoff curring flows. Channel changes caused di- stream reaches of the depositional zone was regime of the Colorado Front Range is char- rectly by this flood, however, were quite slower because there was continued supply of acterized by moderate flows that persist for variable. Intermittent erosion and deposition sediment from eroding reaches upstream and some time. Under this hydrologic regimen, occurred along the entire length of Roaring because there was time for gravel-armor lay- flows greater than two times the mean annual River from Lawn Lake to its confluence with ers to form on the stream bed. Sediment in flood are improbable, and it is unlikely that Fall River, deposition of a massive alluvial fan these armor layers could still be moved by erosion on Roaring River or the alluvial fan occurred where the two rivers join, and vir- moderate flows, however, and by 1987, 80% will be re-initiated. This does not mean that tually no channel changes occurred in the to 90% of the material had been eroded from morphologic adjustments on Fall River will 4-km reach of Fall River in Horseshoe Park. the depositional zone. A 4-yr time series of cease altogether, because there will continue The variable patterns of erosion and deposi- bed-load measurements indicates that sedi- to be small but measureable amounts of sed- tion caused by the flood were due to changes ment eroded from the depositional zone was iment transport. in shear stress and sediment-transport capac- transported steadily through the sinuous ity associated with variations in average gra- reaches of Fall River in lower Horseshoe dient and valley width. Park. Although much higher than natural sed- Spatial and Temporal Discontinuity Geomorphic changes on Fall River under iment transport rates, the increase in sedi- subsequent snowmelt discharges provide a ment load within this reach caused little chan- Thé different reaches of Fall River were sharp contrast between the direct and indirect nel change, mostly because here Fall River is never in the same phase of adjustment. Dur- effects of a catastrophic flood. In 1983, the bounded by very well vegetated banks. The ing the period when erosion was occurring in first year after the Lawn Lake flood, —15.5 x complex history of erosion and deposition in the alluvial-fan reach, deposition was occur- 103 Mg of bed load was transported from the this gravel-bed river illustrates that floods in ring in lower Horseshoe Park. Subsequently, upper part of the study area; this represents small drainage basins may produce highly the same upstream to downstream progres- the equivalent of perhaps 300 yr of sediment variable geomorphic responses. Geological Society of America Bulletin, May 1993 669 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/5/657/3381740/i0016-7606-105-5-657.pdf by guest on 29 September 2021 J. PITLICK ACKNOWLEDGMENTS Collins, B. D., and Dunne, T., 1986, Erosion of tephra from the 1980 Parker, G., 1979, Hydraulic geometry of active gravel rivers: Journal eruption of Mount St. Helens: Geological Society of America of the Hydraulics Division, American Society of Civil Engi- Bulletin, v. 97, p. 896-905. neers, v. 105, p. 1185-1201. Costa, J. E., 1987, Hydraulics and basin morphometry of the largest Pickup, G., Higgins, R. J., and Grant, I., 1983, Modelling sediment Financial support for this research was flash floods in the conterminous United States: Journal of transport as a moving wave—The transfer and deposition of provided by the National Park Service and Hydrology, v. 93, p. 313-338. mining waste: Journal of Hydrology, v. 60, p. 281-301. Gilbert, G. K., 1917, Hydraulic-mining debris in the Sierra Nevada: Pitlick, J., 1988, The response of coarse-bed rivers to large floods in the U.S. Army Research Office under grant U.S. Geological Survey Professional Paper 105, 154 p. California and Colorado [Ph.D. thesis]: Fort Collins, Colo- Griffiths, G. A., 1979, Recent sedimentation history of the Wai- rado, Colorado State University, 137 p. number DAAG29-85-K-0108. My sincerest makariri River, New Zealand: New Zealand Journal of Hy- Pitlick, J., 1993, Sediment routing in tributaries of Redwood Creek, thanks to all those who assisted me in the drology, v. 18, p. 6-28. in Nolan, K. M., Kelsey, H. M., and Marron, D. C., eds., Hack, J. T., and Goodlett, J. C., 1960, Geomorphology and forest Geomorphic processes and aquatic habitat in the Redwood field. I would also like to thank M. D. Harvey ecology of a mountain region in the Central Appalachians: Creek basin, northwestern California: U.S. Geological Sur- U.S. Geological Survey Professional Paper 347, 66 p. vey Professional Paper 1454 (in press). and S. A. Schümm for their advice and en- Harbor, J. M., 1984, Terrestrial and lacustrine evidence for Holo- Schumm, S. A., 1969, River metamorphosis: Journal of the Hy- couragement. P. W. Birkeland, T. N. Caine, cene climatic/geomorphic change in the Blue Lake and Green draulics Division, American Society of Civil Engineers, v. 95, Lakes valleys of the Colorado Front Range [M.A. thesis]: p. 255-273. R. D. Jarrett, A. J. Miller, and K. M. Nolan Boulder, Colorado, University of Colorado. Schumm, S. A., and Lichty, R. W., 1963, Channel widening and Jarrett, R. D., 1990, Paleohydrological techniques used to define floodplain construction along Cimarron River in Southern offered helpful comments on an earlier ver- the spatial occurrence of floods: Geomorphology, v. 3, Kansas: U.S. Geological Survey Professional Paper 352-D, sion of the manuscript. p. 181-195. p.71-88. Jarrett, R. D., and Costa, J. E., 1986, Hydrology, geomorphology Stewart, J. H., and LaMarche, V. C., 1967, Erosion and deposition and dam-break modeling of the July 15,1982 Lawn Lake Dam in the flood of December 1964 on Coffee Creek, Trinity REFERENCES CITED and Cascade Lake Dam failures, Larimer County, Colorado: County, California: U.S. Geological Survey Professional Pa- U.S. Geological Survey Professional Paper 1369, 78 p. per 422-K, 22 p. Andrews, E. D., 1984, Bed-material entrainment and hydraulic ge- Kelsey, H. M., 1980, A sediment budget and an analysis of geo- Williams, G. P., and Wolman, M. G., 1984, Downstream effects of ometry of gravel-bed rivers in Colorado: Geological Society morphic process in the Van Duzen River Basin, northcoastal dams on alluvial rivers: U.S. Geological Survey Professional of America Bulletin, v. 95, p. 371-378. California, 1941-1975: Geological Society of America Bulle- Paper 1286, 83 p. Baker, V. R., and Costa, J. C., 1987, Flood power, in Mayer, L., and tin, v. 91, p. 1119-1216. Wolman, M. G., and Eiler, J. P., 1958, Reconnaissance study of Nash, D., eds., Catastrophic flooding: Boston, Massachu- Lisle, T. E., 1982, Effects of aggradation and degradation on pool- erosion and deposition produced by the flood of August 1955 setts, Allen and Unwin, p. 1-21. riffle morphology in natural gravel channels, northwestern in Connecticut: American Geophysical Union Transactions, Baker, V. R., Kochel, R. C., and Patton, P. C., 1988, Flood geo- California: Water Resources Research, v. 18, p. 1643-1651. v. 39, p. 1-14. morphology: New York, Wiley-Interscience, 503 p. Madej, M. A., 1993, Recent changes in channel-stored sediment, Wolman, M. G., and Gerson, R., 1978, Relative scales of time and Beschta, R. L., 1983, Long-term changes in channel widths of the Redwood Creek, California, in Nolan, K. M., Kelsey, H. M., effectiveness of climate in watershed geomorphology: Earth Kowai River, Torlesse Range, New Zealand: New Zealand and Marron, D. C., eds., Geomorphic processes and aquatic Surface Processes, v. 3, p. 189-208. Journal of Hydrology, v. 22, p. 112-122. habitat in the Redwood Creek basin, northwestern California: Beven, K., and Carling, P., 1989, Floods: Their hydrological, sed- U.S. Geological Survey Professional Paper 1454 (in press). imentological and geomorphological implications: Chi- Miller, A. J., 1990, Flood hydrology and geomorphic effectiveness chester, England, Wiley and Sons, 290 p. in the central Appalachians: Earth Surface Processes and Blair, T. C., 1987, Sedimentary processes, vertical stratification Landforms, v. 15, p. 119-134. sequences and geomorphology of the Roaring River alluvial Newson, M. D., 1980, The geomorphological effectiveness of fan, Rocky Mountain National Park, Colorado: Journal of floods—A contribution stimulated by two recent events in Sedimentary Petrology, v. 57, p. 1-18. Mid-Wales: Earth Surface Processes, v. 5, p. 1-16. Caine, T. N., 1986, Sediment movement and storage on alpine slopes Osterkamp, W. R., and Costa, J. E., 1987, Changes accompanying in the Colorado Rocky Mountains, in Abrahams, A., ed., an extraordinary flood on a sand-bed stream, in Mayer, L., MANUSCRIPT RECEIVED BY THE SOCIETY MAY 21,1992 Hillslope processes: Boston, Massachusetts, Allen and Un- and Nash, D., eds., Catastrophic flooding: Boston, Massa- REVISED MANUSCRIPT RECEIVED OCTOBER 22,19?2 win, p. 115-137. chusetts, Allen and Unwin, p. 201-224. MANUSCRIPT ACCEPTED NOVEMBER 3,1992 Printed in U.S.A. 670 Geological Society of America Bulletin, May 1993 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/5/657/3381740/i0016-7606-105-5-657.pdf by guest on 29 September 2021