Geomorphic response of a headwater channel to augmented flow

Geomorphology 138 (2012) 329–338

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Geomorphic response of a headwater channel to augmented flow Ellen Wohl ⁎, David Dust Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482, USA

a r t i c l e

i n f o

Article history: Received 18 April 2011 Received in revised form 13 September 2011 Accepted 19 September 2011 Available online 24 September 2011 Keywords: Flow regulation Mountain river Step–pool Pool–riffle Flow augmentation

a b s t r a c t Since 1974, flow releases from Long Draw Reservoir have increased annual peak flows on La Poudre Pass Creek, Colorado, from ~ 5.6 m3/s to N 8.4 m 3/s. The creek drains 61 km 2 and channel morphology varies from step–pool to pool–riffle. Comparison of five channel reaches along the creek to channel reaches along neighboring rivers without flow regulation indicates that channel width has increased by as much as a factor of three along La Poudre Pass Creek. Width-to-depth ratio has also increased, the bed material in step–pool channel reaches has coarsened, and residual pool volumes have increased in pool–riffle channel reaches. Pool–riffle channel reaches have undergone the greatest change in response to flow augmentation. Although discharge has increased consistently for all five channel reaches, morphologic response varies in relation to gradient and channel morphology, making it impractical to precisely predict a priori the magnitude of channel response to flow augmentation. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Numerous studies of geomorphic channel response to flow regulation are now available, facilitating syntheses and conceptual models of how factors such as sediment supply, nature of flow alteration, and channel substrate and planform influence channel response (e.g., Ligon et al., 1995; Nilsson and Berggren, 2000; Grant et al., 2003; Burke et al., 2009). With relatively few exceptions, however, existing work focuses on channel response below dams that reduce peak flow (e.g., Hirsch et al., 1990; Magilligan and Nislow, 2001) and on the relatively large channels that are more likely to be impounded behind dams (e.g., Wilcock et al., 1996; Collier et al., 1997). Flow regulation is widespread, however, in mountainous environments (Wohl, 2006). The research summarized here focuses on the response of a relatively small headwater channel to substantial increases in both the magnitude of peak flow and the total volume of annual flow. Montgomery and Buffington's (1997) widely used classification of headwater channels based on bedform sequences proposes that higher gradient channels with step–pool or plane–bed morphology will be less responsive to changes in water and sediment supply than lower gradient channels with pool–riffle morphology because the high gradient channels have larger sediment transport capacities, limited sediment supply relative to transport capacity, greater lateral confinement, and larger bed-forming clasts with higher thresholds for mobility. Montgomery and Buffington (1997) thus refer to high gradient ‘transport’ reaches and low gradient ‘response’ reaches.

⁎ Corresponding author. Tel.: + 1 970 491 5298; fax: + 1 970 491 6307. E-mail addresses: [email protected] (E. Wohl), [email protected] (D. Dust). 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.09.018

Studies of headwater channels in which diversions reduce or eliminate peak flows support this assertion (Madsen, 1995; Graf, 1997; Ryan, 1997; Woods, 2006), with lower gradient channels exhibiting more change in response to reduced flows than do higher gradient channels. More limited work on headwater channels undergoing flow augmentation also indicates that low gradient channel reaches can be very responsive. Highly sinuous channels in the upper Arkansas River basin of Colorado, USA, for example, became less sinuous or even braided in response to peak flows approximately double those of natural peak flows (Dominick and O'Neill, 1998). Channels with augmented flow also exhibited greater bankfull channel width and width-todepth ratios, coarser grain size on bars, and reduced riparian vegetation cover relative to nearby channels without flow augmentation (Abbott, 1976; Dominick and O'Neill, 1998). Montgomery and Buffington (1997) noted that potential responses of step–pool channels to altered water and sediment supply include changes in bedform frequency and geometry, grain size, and pool scour depths although such responses have not been documented to our knowledge except in the case of brief or periodic disturbance associated with debris flows or floods (e.g., Molnar et al., 2008). Potential responses of pool–riffle channels to altered water and sediment supply include changes in channel width, flow depth, and roughness, as well as the changes listed above for step–pool channels (Montgomery and Buffington, 1997). The primary difference between the two channel types is that pool–riffle channels are expected to show greater response to a given change in water and sediment supply and to undergo morphologic changes in response to smaller changes in water and sediment supply. The field area described in this paper provides an ideal situation in which to evaluate the relative responsiveness of differing channel types to substantially augmented flow over a period of multiple decades because of the longitudinal proximity of different channel

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E. Wohl, D. Dust / Geomorphology 138 (2012) 329–338

types and the consistency of other potential controls such as discharge. Long Draw Reservoir, constructed in its present form in 1974, stores up to 13.3 million m 3 of water. The reservoir, which is situated along La Poudre Pass Creek and near the Continental Divide in Colorado, receives flow diverted from the headwaters of the Colorado River on the western slope via Grand Ditch. Water from the reservoir is released into La Poudre Pass Creek as a means of routing the extra water to the Poudre River and downstream consumptive uses (Fig. 1). Diversion structures upstream of Long Draw Reservoir thus have the potential to increase the effective drainage area for La Poudre Pass Creek from 61 km 2 to 111 km 2 (USDA, 2009). A wide, low gradient valley segment immediately downstream from the reservoir has a sinuous, pool–riffle channel. As valley gradient and lateral confinement increase downstream, the channel assumes step–pool morphology. Channel morphology thus varies substantially over a downstream distance of only 7 km, along which only one tributary and no point sources of sediment influence water and sediment yield to the channel. Neighboring channels with a range of drainage areas but no flow regulation provide the opportunity to quantify trends in channel parameters in relation to drainage area, with drainage area being used as a proxy for flow in these ungaged streams. We can then evaluate how La Poudre Pass Creek deviates from these trends. The primary objective of this research is thus to infer the response of La Poudre Pass Creek to augmented flow that has been sustained over three decades. Inference is based on an ergodic substitution of space (neighboring, unregulated rivers) for time (La Poudre Pass Creek before flow augmentation). We conceptualize channel response to augmented flow using an expanded version of Lane's balance. Lane (1955) introduced the qualitative expression Qw S α Qs Ds to relate representative water discharge (Qw), bed slope (S), bed-material load (Qs), and bed sediment size (Ds) for a river reach under dynamic equilibrium. Dust and Wohl (in press) expanded Lane's relation to include measures of bedform, crosssectional, and planform geometry as additional degrees of freedom

Q w S∝Q s Ds ðW=dÞ or Q w

! Δz ∝Q s Ds ðW=dÞ P⋅H a

ð1Þ

where W is the channel width, d is the channel depth, Δz is change in bed elevation along the reach, P is sinuosity, and H a is the reach-averaged bedform amplitude.

COLORADO

W 105° 46'

unn a cre med ek

g Lon

3

ir rvo ese wR Dra

0m 325

2

1

Cre ek

3

3 2 2 1 1

4 3

2

1

325 0m

5

Poudre Rive r

Cor ral

La Po Cr udre ee Pa k ss 325 0m

N 40° 30'

. ... ..... . .

325 0m

32 50 m

þ ↓

o

↑

↑

þ

Q w S ∝Q s Ds ðW=dÞ or Q w

Δz P⋅H a

!↓ o

↑

↑

∝Q s Ds ðW=dÞ

ð2Þ

This conceptual framework gives rise to two hypotheses regarding the nature of channel response to augmented flow along La Poudre Pass Creek. We hypothesize (H1) that the primary geomorphic channel response to increased flow will be increased channel width, as reflected in an increased width-to-depth ratio. Secondary responses will be increased channel sinuosity or other indicators of reduced slope, bed-material coarsening, and/or increased bedform amplitude, measured as increased residual pool volume. We also hypothesize (H2) that response reaches with lower gradient and pool–riffle bedforms will show greater change relative to undisturbed channel reaches than do transport reaches with higher gradient and step–pool bedforms. The rationale underlying H1 is that, given the very coarse-grained glacial sediment underlying La Poudre Pass Creek, and the relatively stable local base level of the Poudre River, channel widening represents a more likely geomorphic response than channel incision. Additional excess transport capacity is likely to result in winnowing and local erosion of the bed, producing a coarser grain size distribution and enhanced pool scour. H2 is essentially a test of relative responsiveness by channel type as predicted and observed in previous studies. The importance of testing these hypotheses lies in their implications for stream management and restoration. Better understanding of which portions of a headwater channel network are most sensitive to changes in water and sediment yield, and of how those sensitive channel segments will respond, is critical for resource managers attempting to protect and restore the geomorphic integrity of streams (Wohl et al., 2007; Buffington and Tonina, 2009). In the absence of data on the geomorphic characteristics of La Poudre Pass Creek prior to flow augmentation, we tested hypotheses H1 and H2 by comparing the current geomorphic parameters of the creek with those of otherwise analogous sites with no flow alteration. We used the parameters bankfull width (W), bankfull depth (d), bankfull width-to-depth ratio (W/d), sinuosity (P), and residual pool volume (poolvol) as a measure of the bedform amplitude (H a ). 2. Study area

m 3000

N

The expanded Lane's relation provides a conceptual model useful for understanding potential geomorphic responses of La Poudre Pass Creek to flow augmentation. The scenario for La Poudre Pass Creek corresponds to the condition where the water supplied to the reach has in+ creased (Qw ) and the sediment supplied to the reach is relatively low and essentially unchanged (Qso). The expanded Lane's relation represents this scenario with the river adjustments possible to maintain the proportionality being flattening of the bed slope (S ↓), coarsening of the bed material (Ds↑), and/or an increase in the width-to-depth ratio ((W/d) ↑)

Ha gue Cre ek 32 50 m

35 00 m

1 km

Fig. 1. Location map of the study area. Rivers drain north in this view. Land within Rocky Mountain National Park is shaded light gray. Dashed lines indicate major topographic contour intervals. Channel study reaches indicated by small black squares and reach number appears next to each square.

At its junction with the Poudre River, La Poudre Pass Creek drains an area of 61 km 2. The catchment is underlain by Pinedale-age glacial till and Precambrian-age biotite schist and Silver Plume Granite (Braddock and Cole, 1990), and elevation ranges from 3800 m at the drainage divide down to 2940 m at the basin mouth. Land cover varies from unvegetated peaks to subalpine meadows and forest. All of the catchment lies within the Roosevelt National Forest. Climate is strongly seasonal; the majority of the mean annual precipitation of ~850 mm falls as snow, and an early summer snowmelt peak dominates the annual hydrograph. Grand Ditch has been operated since the late nineteenth century. The original Long Draw Reservoir completed in 1929 held ~5.4 million m 3 (4400 acre-ft) of water (USDA, 2009). The reservoir was enlarged in 1974 to its present capacity of 13.3 million m3 (10,800 acre-ft), which has facilitated an increase in peak flow releases below the

E. Wohl, D. Dust / Geomorphology 138 (2012) 329–338

Average monthly discharge (m3/s)

5 existing flow below the dam

4 3 2

estimated native flow into the reservoir

1 0

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Fig. 2. Hydrographs of average monthly flow downstream from Long Draw Reservoir and estimated natural flows. Data from USDA (2009).

reservoir from ~5.6 m3/s to N8.4 m 3/s (USDA, 2009). Estimated native water yield for the La Poudre Pass Creek watershed is 1 million m 3 (820 acres-ft). Water imports are about 22.8 million m 3 (18,500 acres-ft), more than tripling the native water yield (USDA, 2009). Inflow from Grand Ditch into Long Draw Reservoir typically begins in May, but stored water from the reservoir can be released into La Poudre Pass Creek as early as April. Releases from the reservoir are stopped when more than 6.2 million m3 (5000 acres-ft) remain in the reservoir during average years, but the reservoir has been drawn down to 1.5 million m3 (1200 acres-ft) during dry years (USDA, 2009). Releases can last into October, but typical operations release high volumes during June through August. Flows into the creek cease in late October or early November (Fig. 2). In the absence of upstream and tributary inputs, much of the creek becomes dry during the winter and early spring and shallow pools can freeze to the bed. Sediment is not released from Long Draw Reservoir, so sediment yield to the study reaches along La Poudre Pass Creek has decreased from natural levels. Sediment yield was likely relatively low prior to construction of the reservoir. The crystalline rocks underlying the basin weather slowly in the cold, dry climate and the relatively gentle sideslopes in the vicinity of the study reaches do not show evidence of large or frequent mass movements. Unregulated streams in the region typically have low suspended sediment concentrations and low rates of bedload transport (Williams and Rosgen, 1989). Characteristics of stream reaches in nearby catchments also examined in this study are summarized in Table 1. Hague Creek is tributary to the Poudre River from the east and upstream from La Poudre Pass Creek (Fig. 1). Hague Creek drains ~38 km2, all of which lies within Rocky Mountain National Park. Corral Creek is tributary to La Poudre

331

Pass Creek from the west just upstream from La Poudre's junction with the Poudre River and drains 17.7 km2 within the Roosevelt National Forest. The Poudre River is the primary stream in the region; it heads in Rocky Mountain National Park at 3270 m elevation and then flows downstream through the Roosevelt National Forest en route to its junction with the South Platte River on the western Great Plains at 1400 m. The Poudre River drains 156 km2 at its junction with La Poudre Pass Creek. Flow in the Poudre River is heavily regulated, starting with the augmented flow entering the river via La Poudre Pass Creek. The upstream portion of the Poudre River, which includes the channel segments examined here, is designated as a Wild and Scenic River with no flow regulation. All of the channel reaches examined in this study lie between 3080 and 2940 m elevation in the subalpine vegetation zone, are underlain by Pinedale till and crystalline Precambrian rocks, and have a snowmelt-dominated hydrograph. We classified channel reaches as step– pool, plane–bed or pool–riffle types (Montgomery and Buffington, 1997) based on dominant bedforms. All channel reaches have a composite bank of cohesive sediment underlain by noncohesive cobble to boulder size sediment. Meadow vegetation of grasses, sedges, and shrubby willows (Salix spp.) grows on stream banks along the pool–riffle channels. These types of vegetation are also present beside the step– pool and plane–bed reaches, along with coniferous trees (Fig. 3). 3. Methods 3.1. Field methods We chose 14 channel reaches, each defined as having consistent gradient and morphology, to characterize the range of channel geometry present in the field area. Our sample design was based on choosing at least one reach of low gradient (pool–riffle morphology) and high gradient (plane–bed or step–pool morphology) along each of the four primary streams. Each channel reach varied from 50 m long for the smallest channels to 250 m long for the larger channels. For each reach, we surveyed thalweg and bankfull gradient and five representative cross sections using a laser theodolite. We measured bed grain size distribution by sampling 100 clasts over intervals of 25 cm along transects perpendicular to flow. We measured residual pool volume by sampling pool area and depth on a grid for 1–2 representative pools in each reach (Lisle and Hilton, 1992). We derived the reach-scale and cross-sectional geomorphic parameters in Tables 1 and 2 from these basic field measurements. 3.2. Evaluation of aerial photographs To assess the response of the La Poudre Pass Creek study reaches to flow augmentation in terms of changes in planform, we compared

Table 1 Drainage area and reach-scale geomorphic and hydrologic parameters for the study sitesa. Study site or channel reach

A (km2)

S (m/m)

D16 (mm)

D50 (mm)

D84 (mm)

Poolvol (m3)

Sinuosity (P)

Q2 (m3/s)

Flow regime

Channel morphology

La Poudre La Poudre La Poudre La Poudre La Poudre Corral 1 Corral 2 Corral 3 Hague 1 Hague 2 Hague 3 Poudre 1 Poudre 2 Poudre 3

24.1 24.6 24.7 24.8 60.8 17.6 17.2 14.3 37.8 37.8 37.5 93.1 93.9 94.2

0.003 0.003 0.043 0.061 0.037 0.034 0.083 0.006 0.045 0.019 0.002 0.004 0.017 0.013

7 22 113 73 59 50 62 7 90 52 12 45 53 50

36 44 245 300 170 122 155 36 197 165 24 100 132 150

68 74 452 635 355 245 435 64 420 305 42 205 270 315

55.6 112.8 1.0 3.1 0.9 0.4 4.2 – 1.8 0.5 6.2 – 3.2 1.3

1.9 2 1 1 1 1 1 1.7 1 1 1.7 1 1 1

9.5 9.5 9.5 9.5 14.6 6.1 6.1 6.1 4.1 4.2 3.8 13.4 13.5 13.5

Augmented Augmented Augmented Augmented Augmented Unregulated Unregulated Unregulated Unregulated Unregulated Unregulated Unregulated Unregulated Unregulated

Pool–riffle Pool–riffle Step–pool Step–pool Step–pool Step–pool Step–pool Pool–riffle Step–pool Plane–bed Pool–riffle Plane–bed Step–pool Plane–bed

a

Pass Pass Pass Pass Pass

1 2 3 4 5

A is drainage area; S is reach-scale thalweg gradient; Poolvol is residual pool volume; Q2 is the peak flow rate with a 2-year return frequency.

332

E. Wohl, D. Dust / Geomorphology 138 (2012) 329–338 Table 2 Cross-sectional geomorphic parameters for the study sitesa. Study site or channel segment

x-sec ID

W (m)

d (m)

W/d (m/m)

ω* (W/m2)

La Poudre Pass 1

1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5

8.6 19.0 29.4 9.0 18.1 15.5 13.9 11.2 14.8 29.7 8.4 9.4 9.5 10.9 9.9 11.2 11.1 11.3 12.3 13.0 14.7 16.5 15.3 15.0 18.0 3.3 4.2 3.9 3.4 5.0 3.4 4.4 4.8 6.4 5.4 4.6 8.3 7.6 4.7 5.7 5.3 5.7 5.6 7.3 7.1 10.8 9.3 7.7 9.8 10.9 15.5 15.0 13.7 12.6 13.3 13.4 17.7 17.4 15.5 13.5 14.9 15.8 17.3 13.3 14.4 15.5 15.0 13.7 12.6 13.3

1.70 1.28 1.23 1.18 1.57 0.98 0.77 1.09 1.10 1.69 1.21 1.37 1.19 0.99 1.08 0.79 1.16 1.09 1.02 0.88 1.02 1.33 0.88 1.27 0.93 0.54 0.65 0.65 0.64 0.53 0.83 0.54 0.71 0.80 0.66 0.98 0.56 0.68 0.90 0.95 1.12 1.06 1.01 0.84 0.89 0.71 0.68 0.82 0.66 0.60 0.81 0.89 0.91 0.82 0.77 1.28 0.79 1.02 1.07 1.54 1.15 0.94 0.84 0.87 0.73 0.81 0.89 0.91 0.82 0.77

5.1 14.9 23.9 7.7 11.5 15.8 18.1 10.3 13.4 17.6 7.0 6.8 8.0 11.0 9.1 14.1 9.5 10.3 12.1 14.7 14.4 12.4 17.3 11.8 19.3 6.1 6.4 6.0 5.3 9.4 4.0 8.1 6.7 8.0 8.1 4.7 14.9 11.2 5.2 6.0 4.7 5.4 5.6 8.7 8.0 15.2 13.6 9.4 14.9 18.1 19.1 16.8 15.1 15.3 17.3 10.5 22.3 17.2 14.4 8.7 13.0 16.7 20.6 15.2 19.8 19.1 16.8 15.1 15.3 17.3

32.5 14.7 9.5 31.0 15.5 18.1 20.1 24.9 19.0 9.4 476.6 428.4 422.1 368.0 405.7 508.0 514.3 504.6 462.7 438.6 360.1 321.2 347.1 353.6 294.7 617.1 484.1 516.2 596.0 410.9 1479.5 1119.9 1039.4 774.3 926.8 78.6 43.1 47.1 76.4 63.1 350.9 323.1 329.2 254.6 260.4 70.5 82.1 98.7 78.3 70.3 12.6 13.0 9.0 7.6 9.9 33.9 35.1 38.3 41.8 39.4 167.5 127.4 129.1 145.2 167.2 115.6 108.8 99.4 129.5 119.6

La Poudre Pass 2

La Poudre Pass 3

La Poudre Pass 4

La Poudre Pass 5

Corral 1

Corral 2

Corral 3

Hague 1 Fig. 3. (A) Upstream view of lower portion of La Poudre Pass Creek reach 3 at 4.5 m3/s flow. (B) Upstream view of upper portion of La Poudre Pass Creek reach 3 at 0.7 m3/s flow. Channel is approximately 9 m wide in both views.

aerial photographs dating from 1958 and from 2005, as available from the USGS EarthExplorer Web application (http://edcsns17.cr.usgs.gov/ NewEarthExplorer/). Although historical aerial photographs are available for the study area for other time intervals starting in the 1920s, many of these photographs lack sufficient resolution for discerning changes in planform along La Poudre Pass Creek. The earliest historical photographs readily available with sufficient resolution for our investigations were taken in 1958, after the construction of the original Long Draw Reservoir in 1929 and prior to enlargement of the reservoir in 1974. Comparison of the 1958 and 2005 aerial photographs provided a means to assess the changes in sinuosity of La Poudre Pass Creek over the past ~ 50 years.

Hague 2

Hague 3

Poudre 1

Poudre 2

3.3. Analyses To evaluate the stability of the La Poudre Pass Creek sites, we used a state diagram approach with a form of specific stream power and width-to-depth ratio as the state and channel geometry metrics, respectively (Dust and Wohl, 2010). Specific stream power is defined as follows ω¼

ρgQSf Wt

ð3Þ

Poudre 3

a W is the bankfull channel width; d is the bankfull channel depth; W/d is the ratio of bankfull channel width to depth; ω* is a form of specific stream power (Eq. (4)) that is used as a state metric in Fig. 8.

E. Wohl, D. Dust / Geomorphology 138 (2012) 329–338

where ρ is the density of water, g is the acceleration of gravity, Q is the flow rate, Wt is the top width of flow, and Sf is the friction or energy grade line slope. Direct application of Eq. (3) to channels with step–pool and pool– riffle bedforms is challenging, because these channels have discontinuous, rapidly varied flow profiles as a prominent feature and, as a result, have a highly varying energy grade line slope within a reach. We use the following form of specific stream power as a state metric for comparison purposes

where Q2 is the peak flow rate with a 2-year return frequency, Wbf is the bankfull top width, and S b is the average bed slope for the reach. Values of Q2 were estimated for the study sites with unregulated flow using the regional regression equation via the Web application StreamStats developed and implemented by the U.S. Geological Survey (Ries et al., 2004). The regional regression equation used in StreamStats to compute Q2 appropriate for the study area is as follows (Capesius and Stephens, 2009) Q2 ¼ 10

ω ¼

ρgQ2 S b Wbf

Bankfull channel width (m)

−2:05 0:78 0:17 2:10 A Sbasin Pi

ð5Þ

where Q2 is the peak flow rate with a 2-year return frequency (ft 3/s), A is the drainage basin area (mile 2), and Sbasin is the mean basin slope

ð4Þ

A

333

35 Pool-riffle sites with augmented flow: La Poudre Pass 1-2

30

Step-Pool sites with augmented flow: La Poudre Pass 3-5

25 20 15 10 5

Sites with unregulated flow

Logistic regression line Probability = 50% W= 0.1414 A + 5.507

Corral 1 Corral 2 Corral 3 Hague 1 Hague 2 Hague 3 La Poudre Pass 1 La Poudre Pass 2 La Poudre Pass 3 La Poudre Pass 4 La Poudre Pass 5 Poudre 1 Poudre 2 Poudre 3 Probability = 50%

0 20

0

40

60

80

100

A (km2)

B 1.80

Pool-riffle sites with augmented flow La Poudre Pass 1-2

1.60

Bankfull depth (m)

1.40 1.20 1.00 0.80 0.60 0.40 0.20

Step-pool sites with augmented flow La Poudre Pass 3-5

0.00 0

10

20

30

40

50

60

70

80

90

Corral 1 Corral 2 Corral 3 Hague 1 Hague 2 Hague 3 La Poudre Pass 1 La Poudre Pass 2 La Poudre Pass 3 La Poudre Pass 4 La Poudre Pass 5 Poudre 1 Poudre 2 Poudre 3

100

A (km2)

C 30

Pool riffle sites with augmented flow La Poudre Pass 1-2 Step-pool sites with augmented flow La Poudre Pass 3-5

25

W/d

20 15 10 5

Corral 1 Corral 2 Corral 3

Hague 1 Hague 2 Hague 3 La Poudre Pass 1 La Poudre Pass 2 La Poudre Pass 3 La Poudre Pass 4 La Poudre Pass 5 Poudre 1 Poudre 2 Poudre 3

0 0

10

20

30

40

50

60

70

80

90

100

A (km2) Fig. 4. (A) Bankfull channel width plotted against drainage area for surveyed cross sections along La Poudre Pass Creek and sites without flow augmentation in the vicinity of Long Draw Reservoir. (B) Bankfull channel depth versus drainage area for La Poudre Pass Creek and analogous sites without flow augmentation in the vicinity of Long Draw Reservoir. (C) Bankfull width-to-depth ratio versus drainage area for La Poudre Pass Creek and analogous sites without flow augmentation in the vicinity of Long Draw Reservoir.

334

E. Wohl, D. Dust / Geomorphology 138 (2012) 329–338

(%), and Pi is the mean annual precipitation (in.). For the augmented flow sites, we increased A to reflect the extra drainage area effectively diverted to La Poudre Pass Creek. 4. Results Downstream hydraulic geometry relations suggest that bankfull channel width should vary as the square root of discharge (Leopold and Maddock, 1953; Park, 1977). Although mountain streams with highly resistant boundaries may not exhibit strong correlations between channel width and discharge (Wohl, 2004), the stream sites with unregulated flow show a progressive overall increase in channel width with respect to drainage area (used here as a surrogate for discharge) (Fig. 4A). We used binary logistic regression techniques (Menard, 1995; Dust and Wohl, 2010) to define a threshold line between the sets of data points corresponding to sites with augmented flow and those with unregulated flow (Fig. 4A). Almost all of the data points for the La Poudre Pass Creek sites lie above this threshold line and have values of channel width up to ~3 times as wide as analogous sites. Fig. 4A and the results from the 2-sample t-tests provided in Table 3 (p = 0.002 and p b 0.001 for pool–riffle and step–pool channels, respectively) suggest that the bankfull width of La Poudre Pass Creek has increased significantly in response to flow augmentation and that the increase in bankfull width is discernibly more pronounced for the pool–riffle reaches than for the step–pool reaches. In terms of the expanded Lane's relation, this can be represented as W ↑↑ for pool–riffle reaches and W ↑ for step–pool reaches. Similarly, Fig. 4B and the results of the 2-sample t-test (Table 3; p = 0.034) indicate that the bankfull depths for the pool–riffle sites along La Poudre Pass Creek are discernibly greater than those of the pool–riffle sites with unregulated flow (Corral 3 and Hague 3), which we represent as d ↑ in terms of the expanded Lane's relation. Bankfull depths for the step–pool sites along La Poudre Pass Creek are also discernibly greater than those of the step–pool sites with unregulated flow (Corral 1–2, Hague 1, and Poudre 2). The width-to-depth ratios for La Poudre Pass Creek, however, are not discernibly different than the relatively wide range of values observed for analogous sites with no flow regulation (Fig. 4C, Table 3, p = 0.052 and p = 0.983 for pool–riffle and step–pool channels, respectively). When interpreting these results, it is important to recognize that small increases in width-to-depth ratios translate into relatively large decreases in dimensionless boundary shear stress and corresponding sediment transport capacity for the range of width-to-depth ratios observed

for La Poudre Pass Creek (W/d b 25) (Dust and Wohl, submitted for publication). Consequently, channel adjustment along La Poudre Pass Creek does not require substantial increases in the width-to-depth ratio. This may explain why increases in width-to-depth ratios along La Poudre Pass Creek are not discernable in Fig. 4C or 2-sample t-tests (Table 3), even though the increases in bankfull widths suggest that the widthto-depth ratios for the La Poudre Pass sites have increased at least slightly. We represent this indiscernible, but probable, increase in width-todepth ratio as (W/d)↑≈, in terms of the expanded Lane's relation. In summary, Fig. 4A and the results of the corresponding t-tests (Table 3) indicate that the geomorphic response of La Poudre Pass Creek to flow augmentation is highly pronounced increases in channel width at the pool–riffle sites, with less pronounced increases in channel width at the step–pool sites. Fig. 4B and the results of the corresponding t-tests (Table 3) indicate that the bankfull depth increases at the pool–riffle and step–pool sites are less pronounced. We represent these geomorphic responses in the cross-sectional geometry of La Poudre Pass Creek to flow augmentation as (W ↑↑/d ↑) ↑≈ for the pool–riffle sites and as (W ↑/d ↑) ↑≈ for the step–pool sites. Three commonly used measures of bed grain size (D16, D50, D84), when plotted against average bed gradient (Fig. 5), suggest that the finer portion of the bed grain size distribution varies little as a result of flow augmentation; D16 does not differ substantially among the sites. The values of D50 and D84 are larger at site LPP 4, the steepest site on LPP Creek, than at unregulated sites, but the size difference is less than a factor of two (Fig. 54). D84 values for the pool–riffle sites along La Poudre Pass Creek are not discernibly different than the analogous sites without flow augmentation (Table 3: t-test p = 0.359), but the step–pool sites for La Poudre Pass Creek (La Poudre Pass 3–5) have discernibly higher D84 values than the analogous sites (Table 3: t-test p = 0.047). In terms of the expanded Lane's relation, we represent these inferred changes in bed material size of La Poudre Pass Creek to flow augmentation as Ds≈ for the pool–riffle sites and as Ds↑ for the step–pool sites. Plots of residual pool volume versus bed gradient (Fig. 6) and the 2sample t-test (Table 3: p b 0.001) indicate that the two pool–riffle sites for La Poudre Pass Creek (La Poudre Pass 1–2) have discernibly higher residual pool volumes than all other sites. The residual pool volumes for the step–pool sites along La Poudre Pass Creek are not discernibly different than the analogous sites without flow augmentation (Fig. 6 and the 2-sample test, Table 3: p = 0.956). We interpret changes in residual pool volumes to be proportional to, and indicative of, changes in bedform amplitude (H a ), because the depth of the residual pool is a

Table 3 Results of the 2-sample t-tests of the geomorphic parameters for the sites with augmented flow versus analogous sites with unregulated flowa. Geomorphic parameter

Channel morphology

Flow regime

n

Mean

Std Dev

Std Error

Dif. in means

t-test p-value

Results

W* (m) W* (m) W* (m) W* (m) d (m) d (m) d (m) d (m) W/d W/d W/d W/d D84 (mm) D84 (mm) D84 (mm) D84 (mm) Log(Poolvol Log(Poolvol Log(Poolvol Log(Poolvol

Pool–riffle Pool–riffle Step–pool s–p + p–b Pool–riffle Pool–riffle Step–pool s–p + p–b Pool–riffle Pool–riffle Step–pool s–p + p–b Pool–riffle Pool–riffle Step–pool s–p + p–b Pool–riffle All Step–pool s–p + p–b

Augmented Unregulated Augmented Unregulated Augmented Unregulated Augmented Unregulated Augmented Unregulated Augmented Unregulated Augmented Unregulated Augmented Unregulated Augmented Unregulated Augmented Unregulated

10 10 15 35 10 10 15 35 10 10 15 35 2 2 3 7 2 7 3 6

7.97 − 2.34 1.71 − 3.50 1.26 0.93 1.08 0.84 13.8 8.8 11.9 11.8 71 53 481 314 1.90 0.23 0.15 0.13

7.50 1.92 1.33 1.65 0.31 0.33 0.17 0.22 5.50 5.45 3.64 5.31 4.24 15.6 142.0 86.20 0.22 0.45 0.30 0.42

2.400 0.610 0.340 0.280 0.098 0.100 0.044 0.037 1.700 1.700 0.940 0.900 3.000 11.000 82.000 33.000 0.150 0.170 0.170 0.170

10.3

0.002

p b 0.05: discernibly higher

5.2

b0.001

p b 0.05: discernibly higher

0.33

0.034

p b 0.05: discernibly higher

0.24

b0.001

p b 0.05: discernibly higher

5.08

0.052

0.03

0.983

18.0

0.359

167

0.047

p N 0.05: not discernibly different p N 0.05: not discernibly different p N 0.05: not discernibly different p b 0.05: discernibly higher

1.67

0.002

p b 0.05: discernibly higher

0.02

0.956

p N 0.05: not discernibly different

(m3)) (m3)) (m3)) (m3))

a W* = W − (0.1414 A + 5.507) is the bankfull topwidth adjusted by the value of the logistic regression line shown in Fig. 4A; n is the number of observations; Std Dev is the standard deviation; Std Error is the standard error of the means; s–p + p–b represent step–pool and plane–bed morphologies, respectively.

E. Wohl, D. Dust / Geomorphology 138 (2012) 329–338

D16, D50, and D84 (mm)

700

335

Corral 1

Step-pool sites with flow augmentation (D84) La Poudre Pass 4 La Poudre Pass 3 La Poudre Pass 5

600 500

Corral 2 Corral 3 Hague 1 Hague 2 Hague 3

400

La Poudre Pass 1

300 Pool-riffle sites with flow

La Poudre Pass 2

augmentation (D84) La Poudre Pass1-2

200

D84 :

La Poudre Pass 3 La Poudre Pass 4

D50 :

La Poudre Pass 5

100

Poudre 1

D16 : 0 0.0001

Poudre 2

0.0010

0.0100

0.1000

1.0000

Poudre 3

Average bed gradient (m/m) Fig. 5. Commonly used metrics of bed grain size (D16, D50, D84) size versus average bed gradient for La Poudre Pass Creek and analogous sites without flow augmentation in the vicinity of Long Draw Reservoir.

component of the bedform amplitude. We represent these geomorphic responses in terms of bedform amplitude for La Poudre Pass Creek to flow augmentation as H a ↑ for the pool–riffle sites and as H a ≈ for the step–pool sites. Comparison of aerial photographs dating from 1958 and from 2005 indicates that the sinuosity of La Poudre Pass Creek has not discernibly changed at even the pool–riffle sites during the past ~50 years (Fig. 7). Long Draw Reservoir was constructed in 1929 and enlarged in 1974, so the time span covered by the available aerial photography of sufficient resolution to discern sinuosity does not span the first ~29 years of flow augmentation. It is therefore possible that the sinuosity of La Poudre Pass Creek changed as a result of flow augmentation prior to the 1958 photographs. We infer that the changes in sinuosity over the past ~50 years and since the start of flow augmentation have been relatively small and can be represented as P≈ for the pool–riffle and step–pool sites. In summary, and using the basic scenario represented by expression 2, we represent the discernable geomorphic responses of La Poudre Pass Creek to flow augmentation as ð6Þ

Low gradient=pool–riffle sites : þ

Qw

≈

Δz ≈ P ⋅H a ↑

!↓

  o ≈ ↑↑ ↑ ↑≈ ∝ Q s Ds W =d

ð7Þ

High gradient=step–pool sites : þ

Qw

≈

Δz ≈ P ⋅H a ≈

!≈

  o ↑ ↑ ↑ ↑≈ ∝ Q s Ds W =d

Residual pool volume (m 3)

1000

These results support H1; channel widening is the primary geomorphic response of La Poudre Pass Creek to flow augmentation. As shown in Fig. 4A and summarized in expressions 6 and 7, channel width is the geomorphic parameter that is markedly greater at the pool–riffle and step–pool sites along La Poudre Pass Creek relative to otherwise analogous sites with no flow alteration. Secondary geomorphic responses of the creek include less pronounced but discernable increases in bankfull depth (d ↑), bedform amplitude (H a ↑) at the pool–riffle sites, and bed material coarsening (Ds↑) at the step–pool sites. The results also support H2; the lower gradient, pool–riffle reaches of La Poudre Pass show a greater degree of geomorphic response to flow augmentation than the steeper, step–pool reaches. Comparison of the data in Figs. 4–6 and the corresponding expressions 6 and 7 indicate that the geomorphic responses at the pool–riffle sites (La Poudre Pass 1–2) involved discernable changes in cross sectional geometry (W ↑↑ and d↑) and bedform geometry (H a ↑). In comparison, the step– pool sites (La Poudre Pass 3–5) show less pronounced changes in cross sectional geometry (W↑ and d↑) and coarsening of the bed material (Ds↑). A pertinent question to be asked is whether La Poudre Pass Creek is continuing to respond since the enlargement of the reservoir in 1974. In geomorphic analyses documented in the literature, various state diagrams have been employed to assess the stability state and/or bedform thresholds (Parker, 1976; Harvey et al., 1985; Dust and Wohl, 2010). We used a form of specific stream power (Eq. (4)) and width-to-depth ratio as the state and channel geometry metrics, respectively, to compare the state of the La Poudre Pass Creek sites with that of the analogous sites without flow augmentation (Fig. 8).

Pool-riffle sites with flow augmentation: La Poudre Pass 1-2

Corral 1 Corral 2 Hague 1

100

Hague 2

Step-pool sites with flow augmentation: La Poudre Pass 3-5

Hague 3 La Poudre Pass 1

10

La Poudre Pass 2 La Poudre Pass 3 La Poudre Pass 4

1

La Poudre Pass 5 Poudre 2 Poudre 3

0.1 0.000

0.001

0.010

0.100

1.000

Average bed gradient (m/m) Fig. 6. Residual pool volume versus average bed gradient for La Poudre Pass Creek and analogous sites without flow augmentation in the vicinity of Long Draw Reservoir.

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E. Wohl, D. Dust / Geomorphology 138 (2012) 329–338

Fig. 7. Aerial photographs of the pool–riffle reach of La Poudre Pass Creek showing the planform in 1958 and 2005. Flow is toward the upper right in each photograph. Lighter gray is meadow vegetation; darker gray is forest. Pale, narrow line along the left edge of the meadow in each photograph is a foot trail. Wider pale line further to the left in the 2005 photograph is an unpaved road. Dam and spillway are visible at the lower left in the 2005 photograph.

This reveals two key characteristics of the data. First, the sets of data points for the step–pool, plane–bed, and pool–riffle channels exhibit internally consistent trends and are isolated from each other. Second, the data points for La Poudre Pass Creek are dispersed within the data points for the analogous sites with the same bedform. If we assume that the analogous sites are in a state of dynamic equilibrium, this suggests that La Poudre Pass Creek has attained a state of dynamic equilibrium since the enlargement of Long Draw Reservoir in 1974. 5. Discussion 5.1. Channel responses to flow augmentation Flow augmentation resulted in substantial increases in channel width and residual pool volume along pool–riffle channel segments of La Poudre Pass Creek. The magnitude of channel response may reflect the proximity of these channel segments to the reservoir outlet, but the absence of pool–riffle channel segments farther downstream made it impossible to directly evaluate the influence of proximity to the reservoir. 10,000

Two characteristics of the augmented flows in La Poudre Pass Creek likely facilitate bank erosion and channel widening; rapid fluctuations in discharge, and greater flow volume. Rapid drawdown following high stage creates positive pore water pressure that weakens a stream bank by reducing its effective strength, and sustained large flows are more likely to be able to remove any sediment reaching the toe of the bank as a result of different mechanisms of bank failure (Thorne, 1982). Rapid rises and falls in discharge are a notable characteristic of the highly artificial flows along La Poudre Pass Creek (Fig. 9), and flow magnitude and volume increased substantially (Fig. 2). The greater magnitude of channel widening in the pool–riffle reaches, as well as the greater increase in residual pool volume, may reflect the thicker sequence of relatively fine-grained alluvium present in the low gradient valley segment that contains the pool–riffle channel reaches. Although boulders with an intermediate diameter of N50 cm are locally present along the bed and basal portion of the bank in the pool–riffle reaches of La Poudre Pass Creek, the overlying fine-grained alluvium is typically 1–2 m thick. In contrast, the finegrained alluvial upper layer is typically b1 m thick along the step– pool reaches of the creek, with boulders 50–150 cm in diameter

Corral 1

Step-pool sites

Corral 2 Corral 3

ω ∗ (W/m2)

1,000

Hague 1

Plane-bed sites

Hague 2 Hague 3

100

La Poudre Pass 1

Pool-riffle sites

La Poudre Pass 2 La Poudre Pass 3

10

La Poudre Pass 4 La Poudre Pass 5 Poudre 1 Poudre 2

1 1

10

100

Poudre 3

W/d Fig. 8. A form of specific stream power versus the width-to-depth ratio for La Poudre Pass Creek and analogous sites without flow augmentation in the vicinity of Long Draw Reservoir.

E. Wohl, D. Dust / Geomorphology 138 (2012) 329–338

Instantaneous discharge (m3/s)

10 2009 (start 26 May, end 13 October)

8 6 4 2 0

Time

337

summer months can overwinter in the deep residual pools of the pool–riffle reaches, but cannot remain in the relatively shallow pools of the step–pool reaches, which freeze completely during winter. During the ‘dewatered’ winter months, any step–pool and plane– bed reach along the creek is thus devoid of fish habitat. Because bed gradient correlates strongly with channel morphology (Montgomery and Buffington, 1997; Wohl and Merritt, 2005; Wohl et al., 2007; Buffington and Tonina, 2009) and bed gradient can be adequately mapped using digital elevation models (DEMs), the spatial distribution of pool–riffle, plane–bed and step–pool reaches, and associated overwinter habitat for fish, can be readily determined from DEMs. This information can be useful in determining the spatial distribution and proportional extent of changes to overwintering habitat for fish within a subbasin or across a region that includes numerous subbasins with regulated flow.

Instantaneous discharge (m3/s)

12 2010 (start 8 June, end 19 October)

10 8 6 4 2 0

Time Fig. 9. Hydrographs at the Long Draw Reservoir outlet for water years 2009 and 2010. Discharge was measured at 15-minute intervals. Data from the Colorado Division of Water Resources.

underneath. Although the fine-grained alluvium is cohesive and vegetated, it is likely more erodible than the very large boulders exposed in the banks of the step–pool reaches. 5.2. Management implications The changes observed along La Poudre Pass Creek indicate that even step–pool ‘transport’ reaches with relatively nonerosive boundaries can undergo substantial channel change as a result of highly augmented flow. This leads to the question of whether it is possible to predict a threshold value at which channel change is likely to occur in response to changes in flow regime, or the magnitude of channel change in response to augmented flow. The difficulty of accurately predicting entrainment of bed material in coarse-grained, poorly sorted substrate suggests that any attempt to quantitatively predict the threshold for, or magnitude of, bed erosion would be at best a first-order approximation. Similarly, predicting the magnitude of bank erosion or channel widening with any precision does not appear to be feasible given the diverse magnitudes of channel response (Figs. 4A and 6) to the same alteration in flow regime along the five channel reaches of La Poudre Pass Creek. The consistency in peak discharge and variability in width among these five channel segments means that downstream hydraulic geometry relations, for example, are of little use in predicting the expected increase in channel width. At present, we can only predict the direction and relative magnitude of channel response to augmented flow. This level of prediction is nonetheless useful. One of the concerns regarding changes in channel geometry and flow regime at La Poudre Pass Creek is the loss of fish habitat. When discharge releases from Long Draw Reservoir cease during the winter months, fish passage along La Poudre Pass Creek also ceases. Salmonids present during

6. Conclusion Substantially augmented flows along La Poudre Pass Creek have resulted in differential channel change along the 7 km of channel downstream from the reservoir. All channel reaches surveyed for this study have greater bankfull widths than would be expected based on a regional regression of bankfull width in relation to drainage area along unregulated channels. Bankfull depths along the creek are also larger than nearby, unregulated channels. The greater difference in values of width, however, suggests that most of the channel adjustment has occurred through bank erosion. Comparisons of grain size parameters and of residual pool volume among the different channels suggest that the bed along some reaches of La Poudre Pass Creek has coarsened as a result of augmented flows, and localized scour has been enhanced, but there is no evidence of substantial, continuous incision in the form of exceptionally high or unstable stream banks. The greatest deviation from expected values for channel parameters occurs in pool–riffle channel reaches. Differences in width, depth, and residual pool volume relative to values in unregulated channels are greater in pool–riffle channel reaches along La Poudre Pass Creek than in step–pool reaches. This supports the common assumption that pool–riffle reaches are response reaches that are more sensitive to changes in water and sediment yield than transport reaches with step–pool morphology. Control exerted by sitespecific influences of sediment supply and bed and bank erodibility make it difficult to quantitatively predict the magnitude of channel response to augmented flow along a specific stream reach, but the differential channel adjustments documented here suggest that considering channel adjustment at the reach scale (channel lengths of 10 1– 10 2 m with consistent morphology) is more appropriate than attempting to predict or characterize channel change across reaches with differing morphology. Reach-scale gradient correlates strongly with channel morphology (Wohl and Merritt, 2005) and can be readily mapped using digital elevation models (DEMs) (Wohl et al., 2007; Buffington and Tonina, 2009). The greater magnitude of channel response to increased flows in pool–riffle channel segments suggests that strategies for identifying the portions of a watershed most responsive to changes in discharge can start with DEM-based maps of reach-scale gradient that can be used to evaluate the abundance and spatial distribution of pool–riffle and other channel types. Responsive channel segments can then be targeted for mitigation or protection to reduce the influence of enhanced peak flows. Acknowledgments We thank Bob Deibel and Kelly Larkin of the USDA Forest Service for bringing La Poudre Pass Creek to our attention initially and for helping to obtain background data on reservoir operations. The

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comments of Dick Marston and three anonymous reviewers improved the manuscript.

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