Waterfalls on the eastern side of Rocky Mountain National Park, Colorado, USA

Geomorphology 198 (2013) 37–44

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Waterfalls on the eastern side of Rocky Mountain National Park, Colorado, USA Jose A. Ortega a, Ellen Wohl b,⁎, Bridget Livers b a b

Departamento de Geología y Geoquímica, Universidad Autónoma de Madrid, Madrid 28049, Spain 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 20 February 2013 Received in revised form 15 May 2013 Accepted 16 May 2013 Available online 23 May 2013 Keywords: Waterfalls Knickpoints Colorado Mountain rivers Longitudinal profile Bedrock joints

a b s t r a c t We examined 30 waterfalls on the eastern side of Rocky Mountain National Park in Colorado, USA, to evaluate whether drainage area or bedrock properties as reflected in joint characteristics correlate more strongly with the location and characteristics of individual waterfalls. Longitudinal profiles tend to be more concave for larger drainages, to have a smaller proportion of total elevation loss in waterfalls, and to have vertical drops rather than angled or ramp waterfalls: we interpret these trends to indicate greater overall incisional capability for larger catchments. Shape of individual waterfalls and height of drop correlate more strongly with bedrock properties: waterfalls in bedrock lacking prominent vertical joints perpendicular to flow are more likely to have a single drop rather than multiple drops, and taller waterfalls correlate with more widely spaced horizontal joints. Waterfalls also noticeably correspond to resistant bedrock outcrops that form steep segments along hillslopes adjacent to the channel. We interpret these results to indicate that the location and characteristics of waterfalls along headwater streams in the study area reflect primarily a limited ability to incise through more resistant segments of the underlying bedrock. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Knickpoints in the form of waterfalls are one of the most visually arresting indicators that adjustment of river longitudinal profiles can be spatially discontinuous. A knickpoint represents an abrupt vertical discontinuity in the profile, and the rate of knickpoint erosion limits upstream transmission of relative base level change (Berlin and Anderson, 2009). Knickpoint morphology can be diverse, including stepped, buttressed, and undercut forms (Young, 1985). Knickpoints can maintain a constant geometry during upstream retreat (Lamb and Dietrich, 2009) or rotate so that the angle of the knickpoint face with the vertical decreases with time and develops into rapids (Gardner, 1983). Knickpoints are more likely to maintain a steep face in strongly bedded or jointed substrates (Holland and Pickup, 1976; Bishop and Goldrick, 1992; Frankel et al., 2007; Lamb and Dietrich, 2009). Knickpoints have been interpreted as reflecting a transient response to base level fall (e.g., Crosby and Whipple, 2006), as well as limited ability to incise through more resistant bedrock (e.g., Miller, 1991): these alternatives are not mutually exclusive. Knickpoints that form where a particularly resistant material outcrops in the channel bed maintain a strong vertical stability during river incision, in contrast with knickpoints resulting from lowering of relative base level, which migrate upstream at a rate controlled by river discharge (Crosby and Whipple, 2006; Larue, 2008). Despite numerous studies of knickpoint morphology and dynamics, distinguishing the relative importance of discharge versus bedrock ⁎ Corresponding author. Tel.: +1 970 491 5298; fax: +1 970 491 6307. E-mail address: [email protected] (E. Wohl). 0169-555X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geomorph.2013.05.010

erosional resistance on knickpoint location and characteristics remains challenging (Phillips et al., 2010). Numerous knickpoints that form waterfalls punctuate the downstream course of rivers draining the eastern side of the continental divide in Rocky Mountain National Park (RMNP), Colorado, USA. Each of the headwater rivers in RMNP is tributary to the Poudre River, Big Thompson River, or North St. Vrain Creek. These major drainages had at least three major episodes of valley glaciation during the Pleistocene Epoch, culminating in the Pinedale glacial period, with glaciers achieving their maximum extent circa 20,000 years ago (Madole et al., 1998; Ward et al., 2009; Dühnforth and Anderson, 2011). Mountain glaciers can effectively deepen and widen valleys, as described in the glacier buzz-saw model (Brozovic et al., 1997), in which glaciated valleys experience more rapid and substantial erosion than do otherwise analogous valleys experiencing only river erosion. Differences in glacial and fluvial erosion are also reflected in persistent differences in valley morphology long after deglaciation (Montgomery, 2002; Amerson et al., 2008). As the Pinedale glaciers in RMNP melted between circa 20,000 and 10,000 years ago, tributary valleys that had been adjusted to the upper level of the ice in the glaciated valleys were left as hanging or oversteepened valleys. This history is evident in the longitudinal profiles of channels tributary to glaciated valleys such as North St. Vrain Creek, where each tributary valley has a relatively low gradient upper portion and then drops precipitously into the main valley, with profile steepening at or just above the level of the Pinedale glacier. Following glacial retreat, each tributary began to incise at a rate partly reflecting its drainage area and discharge. Thus, the contemporary location of many of the waterfalls on the eastern side of RMNP could reflect the rate of

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post-glacial erosion as a function of time since deglaciation and upstream drainage area (here, a surrogate for discharge and incisional capability). Studies in other regions indicate that distance of headwater knickpoint recession can correlate with drainage area (Bishop et al., 2005), particularly for short distances upstream from mainstem–tributary junctions (Crosby and Whipple, 2006). Knickpoint position can also reflect a threshold drainage area for channel incision (Crosby and Whipple, 2006). The relationship between waterfall location and drainage area in RMNP may also be complicated by the fact that at least three episodes of valley glaciation have occurred in the region during the past million years. Waterfall recession might be particularly slow along the small (b 50 km2) headwater streams in RMNP that flow over resistant bedrock, so some of the contemporary waterfalls might reflect continuing adjustment to the earlier Bull Lake (140,000–125,000 years) and pre-Bull Lake (700,000–500,000 years) glacial episodes (Pierce, 2003). Waterfalls can also occur where a river flows over particularly resistant bedrock that slows the rate at which the river can adjust to continuing base level change. Most of the bedrock on the eastern side of RMNP is comprised of crystalline metamorphic or igneous lithologies (Braddock and Cole, 1990) with very similar erosional resistance. Differences in the spacing of joints, however, can create substantial differences in the resistance of the rock to weathering and erosion. Previous work in the region indicates that more closely spaced joints correlate with wider, lower gradient valleys (Ehlen and Wohl, 2002) and with the formation of strath terraces (Wohl, 2008), which led us to hypothesize that differences in joint spacing and geometry might also correlate with the location and characteristics of waterfalls on the eastern side of RMNP. The research summarized here is designed to evaluate the relative influence of drainage area and bedrock properties on continuing adjustment to post-glacial base level fall on headwater streams in mountainous terrain with Pleistocene valley glaciations. We examined watershed-scale adjustment by evaluating whether concavity ratio and total elevation loss in waterfalls correlate with drainage area. We then tested whether drainage area and related variables such as discharge, or bedrock properties as reflected in joint characteristics, correlate more strongly with the location and characteristics of individual waterfalls in Rocky Mountain National Park. Although previous studies such as Miller (1991) and Lamb and Dietrich (2009) acknowledged the importance of bedrock joints in waterfall location and characteristics, the research reported here builds on this work by statistically testing the relative importance of drainage area and joint characteristics in explaining observed variability in height, angle, and shape among individual waterfalls. 2. Study area Rocky Mountain National Park straddles the continental divide between streams draining eastward to the South Platte River and ultimately the Mississippi River, and streams draining westward into the Colorado River. Elevations along the continental divide are 3800 to 4300 m, and the eastern boundary of the park lies at ~2500 m. The park is underlain primarily by Precambrian-age crystalline rock units. The most extensive lithologies are Silver Plume granite and biotite schist (Braddock and Cole, 1990). Most bedrock outcrops are densely jointed, with lesser joints spaced a few centimeters apart and prominent joints b4 m apart. Compressive strength of the crystalline lithologies present in the park averages 50–60 using a Schmidt hammer: differences in joint geometry exert a much stronger influence on bedrock resistance to weathering and erosion (Wohl, 2008). Rocky Mountain National Park lies within the Colorado Front Range, which has been relatively tectonically quiescent since the early Tertiary (Crowley et al., 2002; Anderson et al., 2006). Range crests at 4000 m elevation take the form of narrow, glaciated spines. Widespread surfaces of low relief at 2300–3000 m elevation are deeply incised by fluvial canyons. During the past few million years, incision of the South Platte River has driven exhumation of the Denver basin, which forms the eastern border of the Front Range. Tributaries of the South Platte,

including those examined in this study, continue to incise the crystalline core of the Front Range in response to this exhumation (Anderson et al., 2006); and fluvial longitudinal profiles display an inflection point, with deeper incision downstream from the inflection. The inflection point on each of the major drainages (Poudre, Big Thompson, North St. Vrain) is well downstream from RMNP and our study area. The headwater tributaries examined here experienced proximal base level change associated with Pleistocene valley glaciation, as noted above. Pleistocene terminal moraines lie at elevations between 2590 and 2370 m within RMNP. Of the seven headwater drainages examined in this paper, only two were not glaciated (Table 1). Climate and vegetation within RMNP vary strongly with elevation. Mean annual precipitation decreases from 100 cm at the continental divide down to 36 cm at the eastern boundary of the park (Doesken et al., 2003). Stream flow is dominated by snowmelt runoff that produces an annual peak in late spring and early summer. Peak discharge per unit drainage area does not exceed 1.7 m3/s/km2 (Jarrett, 1993). Alpine vegetation above 3400 m gives way to subalpine forest of spruce, fir, and pine (Picea, Abies, and Pinus spp., respectively) at elevations of 2740 to 3400 m and montane forest of pine and Douglas-fir (Pinus and Pseudotsuga spp., respectively) at 1830 to 2740 m elevation (Veblen and Donnegan, 2005). Valleys in RMNP are longitudinally segmented and vary downstream over lengths of 102–103 m between unconfined valleys of low gradient and relatively wide valley bottom, partly confined valleys, and steep, narrow, confined valleys. Valley segmentation reflects Pleistocene glaciation and spatial variations in joint density (Ehlen and Wohl, 2002), with unconfined valleys typically occurring immediately upstream from Pleistocene terminal moraines and/or having more densely spaced joints (Wohl and Beckman, in press). The waterfalls that we examine here (Fig. 1) occur in confined or partly confined valley segments. Channels within confined valley segments have bedrock or large boulder substrate and cascade or step-pool morphology and are more likely than unconfined valley segments to include bedrock waterfalls. Bedrock is exposed at waterfall locations partly because of high transport capacity within the active channel, although exposed bedrock typically extends beyond the immediate vicinity of the channel; i.e., a ledge or exceptionally steep section of hillslope is commonly at the same elevation as the waterfall that extends laterally for hundreds of meters away from the channel. Channels within partly confined valley segments can have discontinuous bedrock exposure along the active channel, as well as boulder to cobble substrate, and cascade, step-pool, or plane-bed morphology. Our observations of sediment dynamics around channel-spanning logjams during the past few years indicate that cobble-size and finer sediments move each snowmelt season. 3. Methods 3.1. Field methods Streams in RMNP contain numerous small drops, particularly in step-pool reaches; and very tall steps in steep channel segments grade into waterfalls. Vertical drops in the stream that were ≥1 m tall and formed in bedrock were designated as waterfalls. For each of the 30 waterfalls that we characterized, we mapped the location using a handheld GPS with ±3 m horizontal accuracy. We categorized waterfall shape as vertical (free falling water over a vertical lip) or ramp (water flowing down a steeply inclined planar surface formed by exposed bedrock), and measured the angle of ramp waterfalls. We measured the height of the vertical drop and length of ramped falls, as well as channel width upstream and at the base of the falls (Fig. 2). Waterfall height was recorded from the edge of the waterfall lip to the bankfull flow level in the plunge pool. Most waterfalls in RMNP do not have deep plunge pools: depths are b2 m and typically b1 m, at least in part because of very large boulders immediately below the plunging flow. We measured joint characteristics in bedrock exposures at each falls,

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39

Table 1 Characteristics of study sites. Waterfall

Drainage name

Aa (km2)

Slb (m/m)

Q2c (m3/s)

Glaciated

Rhd (m/m)

CRe

Ewf

Copeland 1 Copeland 2 Calypso Ouzel Bluebird 1 Bluebird 2 Bluebird 3 Bluebird 4 Sandbeach 1 Sandbeach 2 Alberta 1 Alberta 2 Glacier Black 1 Black 2 Black 3 Fern Spruce Chasm Black Canyon 1 Black Canyon 2 Black Canyon 3 Black Canyon 4 Black Canyon 5 Black Canyon 6 Black Canyon 7 Black Canyon 8 Bridalveil 1 Bridalveil 2 Bridalveil 3

North St. Vrain Creek

7.6 64.2 19.8 14 3.8 4.1 4.1 6.5 13.2 3.6 21.7 21.5 13.2 5.8 5.9 5.9 7.4 1.6 19.3 14.7 15 15 15 15.1 15.3 15.5 15.5 7.5 7.5 8.8

0.052 0.050 0.291 0.316 0.176 0.176 0.161 0.076 0.168 0.208 0.094 0.176 0.090 0.189 0.155 0.230 0.361 0.303 0.189 0.126 0.172 0.210 0.210 0.244 0.128 0.128 0.185 0.194 0.344 0.138

3.3 12.3 4.6 4.3 0.9 2.1 2.1 3.0 6.0 0.9 8.8 8.8 6.0 3.5 3.5 3.5 6.0 0.8 4.6 2.0 2.1 2.1 2.1 2.1 2.1 2.1 2.1 1.2 1.2 1.3

Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No No No No No No No No No No No

0.052

−0.07

0.009

0.102 0.106

0.004 −0.02

0.035 0.128

0.150

0.004

0.093

a b c d e f g h

Cony Creekg Ouzel Creekg Ouzel Creek Ouzel Creek Ouzel Creek Ouzel Creek Sandbeach Creekg Glacier Creekh Glacier Glacier Glacier Glacier Fern Creekh Spruce Creekh Fall Riverh Black Canyon Creekh Black Canyon Black Canyon Black Canyon Black Canyon Black Canyon Black Canyon Cow Creekh Cow Creek Cow Creek

0.079

−0.06

0.22

0.167 0.163 0.067 0.115

−0.01 −0.02 −0.06 −0.04

0.031 0.007 0.012 0.054

0.104

−0.04

0.033

Drainage area at waterfall. Local stream gradient as measured from topographic maps. Peak discharge with a 2-year recurrence interval, from StreamStats. Relief ratio for mainstem (total elevation loss along river divided by total river length). Concavity ratio (see Fig. 3). Proportion of total elevation loss between headwater lake and downstream reference point that is in waterfalls. Catchments within the North St. Vrain Creek drainage. Catchments within the Big Thompson River drainage.

including spacing (categories of 1 = > 100 cm, 2 = 100 to 30 cm, 3 = b 30 cm), continuity (categories of 1 = no to few continuous, 2 = continuous, no infill, 3 = continuous, infill), and width (categories of 1 = b1 mm, 2 = 1–5 mm, 3 = > 5 mm) of vertical joints parallel and perpendicular to flow and horizontal joints. These categories are based on those developed to describe joints for Selby's (1980) rockmass strength classification. This resulted in nine variables describing joints: spacing, continuity, and width for each of the three sets of joints. 3.2. Analyses With the field-measured GPS coordinates, we calculated drainage area A, local stream gradient S, and peak discharge with a 2-year recurrence interval Q2 using the U.S. Geological Survey's online program StreamStats, which uses regression equations developed by Capesius and Stephens (2009) for calculating discharge at specified recurrence intervals (http://water.usgs.gov/osw/streamstats/colorado.html). Drainage area is computed from a watershed polygon generated using the website's software, and local stream gradient represents gradient calculated for a length of stream typically about 1 km upstream from the waterfall: both are derived from 10-m DEMs. We used Q2 rather than, for example Q100, because of the relatively small difference between approximately annually recurring peak discharges and longer recurrence interval flows: the 100-year flood in the Colorado alpine region is less than two times the mean annual flood (Pitlick, 1994). We distinguish between reach-scale metrics and watershed-scale metrics. Reach-scale metrics include A; S; Q2; joint spacing, continuity, and width; and waterfall angle, shape, and height: of these, waterfall angle, shape, and height are response variables and the other metrics

are potential control variables. Watershed-scale metrics are relief ratio Rh, concavity ratio CR (Fig. 3), and proportion of total elevation loss in waterfalls Ew: of these, CR and Ew are response variables, and A, Q2, and Rh are potential control variables. We used categorical values for joint characteristics and waterfall angle and shape, and continuous values for all other variables. All statistical analyses were conducted using the statistical software R. We ran a correlation test among all continuous variables to evaluate how variables correlate with each other. We used binomial t-tests for two group comparisons, and ANOVA followed by Tukey HSD tests for comparisons of more than two groups on categorical variables (Ott and Longnecker, 2010) to determine whether group means were significantly different for continuous response variables. When ANOVA indicates differences among variables, Tukey HSD tests are used to perform pairwise comparisons to indicate which pairs are significantly different from one another. For each continuous response variable, we created sets of models using linear stepwise regression and using one measure of jointing (e.g., vertical joints parallel to flow or vertical joints perpendicular to flow) at a time with all other control variables. We compared Akaike Information Criterion (AIC) values (Akaike, 1973; Hurvich and Tsai, 1989) to select the model with the best fit, p values, and adjusted R2 values. The AIC values are widely used to compare linear models and to choose the model with the lowest error. We compared p values of individual variables within each model to determine which variables were the most influential for the response variable. For each categorical response variable, we created a set of models using a generalized linear stepwise regression with binomial response. Each set involved using one measure of jointing at a time with all other control variables. We compared AIC values to select the

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105° 30’ W

N

40° 30’ N

rn

uce Spr

Fe

.Denver Rocky Mountain National Park COLORADO

waterfalls erminal moraine terminal

0

8 km

Fig. 1. Location map of waterfalls and glacial terminal moraines in Rocky Mountain National Park (darker gray in inset map and gray shading in larger map), Colorado, USA. Measured waterfalls are indicated by stars. Because some waterfalls are located very close together, the map does not show 30 stars. Creeks along which waterfalls were characterized are labeled. The continental divide is at 3800 to 4300 m elevation, and the eastern boundary of the national park is at about 2500 m elevation.

model with the best fit, and p values of individual variables within each model were compared to determine which models were the most influential for the response variable. Very few control variables were significant for the categorical response variables of waterfall angle and shape, and goodness of fit tests could not be conducted. Therefore, we built classification trees (Everitt and Dunn, 2001) for waterfall angle and shape using all potential control variables and identifying which variables divided the groups most efficiently (i.e., divided the response variable into two categories). The statistical results presented below include only the final model for each of the response variables, with corresponding coefficients and p values. For waterfall angle and shape, the most important variable from the classification tree output is identified. 4. Results Statistical analyses produced a significant model for each response variable. Table 2 lists the control variables that had a significant correlation for each response variable.

4.1. Watershed-scale variables For watershed-scale variables, CR correlates with A; and Ew correlates with A and Q2 (Table 2). Longitudinal profiles become more concave at larger drainage areas and the proportion of total elevation loss in waterfalls declines as drainage area and discharge increase. Although CR values display a general trend with increasing A, it is worth noting the very large range in CR values at small drainage areas (Fig. 4): a consistent pattern does not emerge until drainage area exceeds ~50 km2. Increasing concavity with greater drainage area is expected because previous studies indicated that greater discharge equates to greater erosional capacity and therefore greater ability to develop a concave longitudinal profile despite tectonic uplift or locally resistant bedrock (e.g., Merritts and Vincent, 1989; Crosby et al., 2007). One potential explanation of the large range of CR values at smaller drainage areas is that smaller drainages can be particularly affected by resistant bedrock outcrops that limit the stream's ability to develop a concave profile. Declining proportion of total elevation loss in waterfalls at larger drainage areas is also expected if waterfalls are viewed as indicators of at least

J.A. Ortega et al. / Geomorphology 198 (2013) 37–44

41

Wh = waterfall height Wα = waterfall angle Ew = Wh1 + Wh2/H lake

Waterfall 1

H Waterfall 2

Wh 2

Wα

Pinedale terminal moraine

Fig. 2. Illustration of measured parameters for waterfalls. Parameter Ew is the proportion of total elevation loss in watersheds, where H is the total vertical drop along mainstem channel, typically from an alpine lake to the terminal moraine.

transient inability of the entire longitudinal profile to keep pace with base level fall or the inability to incise more resistant bedrock as rapidly as less resistant bedrock up- and downstream from the waterfall

(Bishop et al., 2005; Crosby and Whipple, 2006; Berlin and Anderson, 2009). Analysis of watershed-scale variables thus indicates that the longitudinal profiles of headwater rivers on the eastern side of RMNP

3800

Cony Creek CR: 0.004

Elevation (m)

3600 3400 3200 3000 2800 2600

- area

0

)/ area

2000

4000

6000

8000

10000

3800

Fall River CR: -0.06

3600 3400

Elevation (m)

CR = (area

3200 3000 2800 2600 2400 0

2000 4000 6000 8000 10000 12000 14000 16000

Distance downstream (m) Fig. 3. Illustration of method for calculating concavity ratio, with two examples from the study area.

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5. Discussion and conclusions

Table 2 Summary of results from statistical analyses using stepwise regression. Parameter

Estimate

Watershed-scale variables Concavity ratio model parameters Intercept Drainage area Proportion of elevation loss in waterfall model parameters Intercept Drainage area Q2 Reach-scale variables Waterfall height model parameters Intercept Slope Joint spacing

Standard error

p-Value

Significance

−0.009 −0.0007

0.007 0.0002

0.216 0.002

0.01

0.221 −0.005 0.023

0.132 0.002 0.008

0.145 0.019 0.032

0.01 0.01

0.0002 0.0004 0.00001

0.001 0.001

32.5 62.9 −42.6

7.25 15.34 7.60

adjust to base level fall and resistant bedrock outcrops in a manner that at least partly reflects drainage area and relative discharge.

4.2. Waterfall characteristics Waterfall angle correlates with A, with a split at drainage area of 6 km2: waterfalls with smaller drainage area tend to be ramps, whereas waterfalls with larger drainage area are more likely to have vertical faces. Waterfall shape correlates with vertical joint spacing perpendicular to flow: waterfalls with no prominent vertical joints perpendicular to flow are more likely to have a single drop, whereas those with more closely spaced joints perpendicular to flow are more likely to have multiple drops. Waterfall height correlates with S and spacing of horizontal joints: taller waterfalls correlate with steeper slopes and more widely spaced horizontal joints. The correlations for waterfall shape and height make intuitive sense: given that quarrying of jointed blocks is a primary mechanism by which bedrock waterfalls are formed and maintained (Whipple et al., 2000; Lamb and Dietrich, 2009; Dubinski and Wohl, 2013), more widely spaced jointed blocks are likely to result in taller waterfalls with a single drop. Analysis of reach-scale variables thus indicates that both drainage area and bedrock properties correlate with waterfall characteristics.

0.04

Concavity Ratio

0.02

0.00

-0.02

-0.04

-0.06

-0.08 0

20

40

60

80

100

120

Drainage area (km2) Fig. 4. Concavity ratio varies widely for smaller drainages, but is consistently negative (indicating a concave longitudinal profile) for drainages >50 km2. Each data point represents a watershed. Plot shows 21 data points because we included every watershed on the eastern side of Rocky Mountain National Park, rather than just the watersheds in which waterfalls were characterized.

The results of the statistical analyses, as expected, suggest that drainage area and bedrock resistance both influence waterfall characteristics. At the scale of an entire watershed, the overall shape of the longitudinal profile and the proportion of total elevation loss within knickpoints likely reflect relative incisional capability among different drainages, as indicated by the correlation between watershed-scale characteristics and drainage area. At the scale of individual waterfalls, incisional capability continues to influence waterfall characteristics, particularly angle of the waterfall face; but the details of bedrock heterogeneities, as reflected in joints exposed at the surface, more strongly influence waterfall shape and height. An important caveat in interpreting the results from RMNP is that we initially expected to compare joint characteristics of bedrock outcrops at waterfalls with characteristics of joints exposed in outcrops along portions of the channel network that did not have waterfalls. Despite the generally high relief and steep terrain in the study area, however, we could not find outcrops along channels large enough to obtain statistically valid samples of joints away from waterfall sites. This suggests that the location of waterfalls does reflect the distribution of more resistant bedrock: as noted in the description of the study area, the longitudinal position of waterfalls in the channel network commonly coincides with unusually steep hillslope segments. The proportion of total elevation loss in waterfalls is inversely proportional to drainage area. This is expected if numerous waterfalls indicate limited ability to incise and A is a surrogate for incisional capability. Waterfall height, however, does not correlate with drainage area. This suggests that the height of individual waterfalls reflects the thickness of particularly resistant bedrock units rather than relative incisional capability of larger versus smaller drainages. Waterfalls are located at more resistant bedrock, and individual falls tend to be taller where joints are more widely spaced and bedrock resistance is greater. These correlations strongly suggest that waterfall location reflects bedrock controls. An additional observation that was not tested statistically also suggests that the specific location of individual waterfalls reflects bedrock characteristics. Only 5 of the 30 waterfalls characterized here – two falls on Ouzel Creek; and 1 each on North St. Vrain Creek, Glacier Creek, and the Fall River – have a distinct downstream gorge that appears to have been formed by progressive knickpoint retreat. These falls are not necessarily located at the largest drainage areas in the data set, but instead appear to reflect the location of particularly thick (along-stream) exposures of massive bedrock. The other waterfalls occur at a pronounced topographic break that is also present in hillslope morphology, so that waterfall location appears to reflect predominantly the landscape position of the resistant bedrock rather than continued fluvial incision through that particular resistant section. A consistent pattern emerges that explains the location and characteristics of waterfalls in RMNP. Larger drainages have greater discharge and presumably have greater incisional capability, as reflected in more concave profiles and a smaller total elevation loss within waterfalls, and are more likely to have waterfalls that are vertical drops rather than ramps. The specific location and morphology of individual waterfalls are strongly influenced by bedrock properties, as reflected in waterfall height and shape. We present a conceptual model that focuses on how the specifics of joint characteristics correlate with waterfall morphology (Fig. 5). Our observations suggest that widely spaced joints limit the efficacy of bedrock quarrying at waterfalls by stream flow (Hancock et al., 1998; Whipple et al., 2000; Chatanantavet and Parker, 2009; Lamb and Dietrich, 2009; Dubinski and Wohl, 2013), helping to promote ramped rather than vertical waterfalls, particularly at smaller drainage areas. This is illustrated by the waterfall on Glacier Creek shown in Fig. 5, where the only well-developed joints are parallel to the flow surface, promoting a ramped surface. As joints become increasingly

J.A. Ortega et al. / Geomorphology 198 (2013) 37–44

Bedrock

Inferred erosional Joint spacing processes least dense

increased efficiency of plucking

Massive (minimal jointing)

most diffuse

Jointed

most dense

most concentrated

43

Example from RMNP

Waterfall morphology broad ramp

Glacier Creek

narrow inner channel, potholes Fall River

single drop, tall waterfall Ouzel Creek

multiple smaller steps

single small step

Cow Creek

North St. Vrain Creek Fig. 5. Conceptual model of the role of joints in waterfall morphology. The descriptive phrases for joint spacing and inferred erosional processes refer to bedrock outcrops along the channel in the immediate vicinity of the waterfall.

more closely spaced, quarrying becomes more likely. The orientation of well-developed or prominent joints also becomes particularly important. Vertical joints parallel to flow may promote inner channels and gorges, as illustrated by Chasm Falls on Fall River (Fig. 5) and facilitate waterfall retreat. Well-developed vertical joints perpendicular to flow promote vertical drops as block quarrying promotes parallel retreat of the waterfall drop, as illustrated by Ouzel Falls on Ouzel Creek (Fig. 5). The more closely spaced these joints are, the more likely the waterfall is to be broken up into numerous small vertical steps. Horizontal joints promote quarrying of blocks from the waterfall lip and also create numerous small vertical steps, as illustrated by Bridalveil Falls on Cow Creek (Fig. 5). Controls on waterfall characteristics in Rocky Mountain National Park present an interesting contrast to the interpretations of Castillo et al. (2013) for knickpoints on the Scottish island of Jura. On Jura, distance of knickpoint retreat scales to the drainage area, and local channel slope and drainage area influence the vertical distribution of knickpoints. Drainage areas smaller than ~ 4 km2 tend to be unable to respond to base level fall and to have convex longitudinal profiles. In the RMNP data set, drainages larger than ~50 km2 tend to be concave, although only 7 of 21 catchments have straight or convex longitudinal profiles (Fig. 4). Smaller catchments show a wide scatter in concavity ratio, but those that are concave tend to become more so with increasing

drainage area. The difference in threshold drainage area between Jura and RMNP may reflect the much drier climate and lower discharge per unit drainage area in Colorado compared to Scotland, although other variables such as differences in bedrock resistance to weathering and erosion between the two regions could complicate this interpretation. We conclude that vertical discontinuities in the longitudinal profile of headwater streams in Rocky Mountain National Park reflect primarily a limited ability to incise through massive, resistant portions of the underlying bedrock. Longitudinal variation in substrate resistance seems to exert a stronger influence on knickpoint location and characteristics than rate of knickpoint retreat in response to relative base level fall following Pleistocene deglaciation. The greater the drainage area and discharge of a stream, the greater the relative ability to maintain a concave longitudinal profile; but even the largest streams examined here have some waterfalls at resistant bedrock outcrops. Systematic examination of the strength of correlations between drainage area and bedrock properties versus waterfall characteristics in other regions would be useful. Such investigation could provide further insight into the relative importance of watershed-scale versus reach-scale controls on knickpoint formation and development of longitudinal profiles in regions undergoing base level change or characterized by differential bedrock resistance. Within a region, reach-scale

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