Waterfalls on the eastern side of Rocky Mountain National Park, Colorado, USA Jose A. Ortega a , Ellen Wohl b, ⁎, Bridget Livers b a Departamento de Geología y Geoquímica, Universidad Autónoma de Madrid, Madrid 28049, Spain b Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482, USA article info abstract Article history: We examined 30 waterfalls on the eastern side of Rocky Mountain National Park in Colorado, USA, to evaluate Received 20 February 2013 whether drainage area or bedrock properties as reflected in joint characteristics correlate more strongly with Received in revised form 15 May 2013 the location and characteristics of individual waterfalls. Longitudinal profiles tend to be more concave for Accepted 16 May 2013 larger drainages, to have a smaller proportion of total elevation loss in waterfalls, and to have vertical Available online 23 May 2013 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 Keywords: Waterfalls with bedrock properties: waterfalls in bedrock lacking prominent vertical joints perpendicular to flow are Knickpoints more likely to have a single drop rather than multiple drops, and taller waterfalls correlate with more widely Colorado spaced horizontal joints. Waterfalls also noticeably correspond to resistant bedrock outcrops that form steep Mountain rivers segments along hillslopes adjacent to the channel. We interpret these results to indicate that the location and Longitudinal profile characteristics of waterfalls along headwater streams in the study area reflect primarily a limited ability to Bedrock joints 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 dis- continuity in the profile, and the rate of knickpoint erosion limits up- stream transmission of relative base level change (Berlin and Anderson, 2009). Knickpoint morphology can be diverse, including stepped, but- tressed, 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 de- creases with time and develops into rapids (Gardner, 1983). Knickpoints are more likely to maintain a steep face in strongly bedded or jointed sub- strates (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 abil- ity to incise through more resistant bedrock (e.g., Miller, 1991): these al- ternatives 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 dynam- ics, distinguishing the relative importance of discharge versus bedrock erosional resistance on knickpoint location and characteristics remains challenging (Phillips et al., 2010). Numerous knickpoints that form waterfalls punctuate the down- stream 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 achiev- ing 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 expe- rience 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 pro- files 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 gla- cial 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 Geomorphology 198 (2013) 37–44 ⁎ Corresponding author. Tel.: +1 970 491 5298; fax: +1 970 491 6307. E-mail address: ellen.wohl@colostate.edu (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 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph