Sources and interpretation of channel complexity in forested subalpine streams of the Southern Rocky Mountains

Abstract

We evaluate correlations between stream geomorphic complexity and characteristics of the adjacent riparian forest, valley geometry, and land use history in forested subalpine streams of the Colorado Front Range. Measures of geomorphic complexity focus on cross-sectional, planform, and instream wood piece and logjam variables. We categorize adjacent riparian forests as old-growth unmanaged forest (OU), younger unmanaged forest (YU), and younger managed forest (YM), and valley geometry as laterally confined, partly confined, or unconfined. Significant differences in geomorphic stream complexity between OU, YU, and YM result primarily from differences in wood pieces and logjams, and these differences correlate strongly with pool volume and organic matter storage. Significant differences in planform and crosssectional complexity correlate more strongly with valley geometry, but do not explain as much of the observed variability in complexity between streams as do the wood variables. Unconfined OU streams have the largest wood loads and the greatest complexity, whereas legacy effects of logging, tie-drives, and channel simplification create lower complexity in YM streams, even relative to YU streams flowing through similarly aged forest. We find that management history of riparian forests exerts the strongest control on reduced functional stream channel complexity, regardless of riparian forest stand age. PUBLICATIONSbut this relationship has been difficult to demonstrate in the field [Palmer et al., 2010]. Although previous studies have related geomorphic complexity of floodplain and instream units to riparian plant species in natural watersheds [Harris, 1988] and watersheds disturbed by human activity [Hupp and Rinaldi, 2007;Gumiero et al., 2015], they do not evaluate such complexity in relation to variations in valley geometry and forest disturbance history. In this paper, we evaluate correlations between different measures of geomorphic complexity in small mountain streams of the Colorado Front Range and characteristics of the adjacent riparian forest, valley geometry, and land use history.Exchanges of water, sediment, and organic matter within and between terrestrial and stream environments influence physical and biological stream dynamics, as well as ecological food webs [Baxter et al., 2005], in turn influencing geomorphic complexity. In this context, riparian forest stand age can exert a particularly important indirect influence on channel complexity by serving as a control on the recruitment of large wood (10 cm diameter and 1 m length) to channels. Trees in old-growth forests (200 years stand age) are larger in diameter and thus greater in volume than trees in younger growth forests. Trees with greater diameter are less likely to be transported downstream in small streams due to the relative dimensions of wood pieces and channels [Braudrick et al.are thus retained close to where they fall in the stream [Lienkaemper and Swanson, 1986;Wohl and Jaeger, 2009] and have greater potential to trap mobile wood and fo...

Historical land use as a driver of alternative states for stream form and function in forested mountain watersheds of the Southern Rocky Mountains

“…Livers and Wohl () found significant differences in physical complexity (spatial heterogeneity) in the form of length of channel, wood load, logjam volume, and pool volume per unit length of valley, and longitudinal variability in bed elevation and bankfull channel width in relation to stream category. Fundamentally, the forms of physical complexity that they evaluated are largely driven by in‐stream wood, with greater wood loads resulting in greater physical complexity.…”

“…In contrast, blowdowns and hillslope mass movements cause fast, direct inputs of LW into streams, with more immediate effects on stream form and function. However, streams with historical land use in the Southern Rockies are significantly wider than the individual channels of their natural, multithread counterparts (Livers and Wohl, ) and lack bridge and ramp pieces that facilitate formation of channel‐spanning logjams (Beckman and Wohl, ) following slow or fast wood recruitment. LW pieces entering managed channels have to be sufficiently long to span the bankfull channel width and initiate logjams that result in the channel avulsions and overbank flow if multithread planform is to develop.…”

“…counterparts (Livers and Wohl, 2016) and lack bridge and ramp pieces that facilitate formation of channel-spanning logjams (Beckman and Wohl, 2014b) following slow or fast wood recruitment. LW pieces entering managed channels have to be sufficiently long to span the bankfull channel width and initiate logjams that result in the channel avulsions and overbank flow if multithread planform is to develop.…”

Floodplain downed wood volumes: a comparison across three biomes

“…We used additional data on downed LW volumes from the montane zone from Jackson and Wohl (2015). Basal areas for floodplain forest adjacent to the channel were measured with a Panama angle gauge in some of the reaches in which downed wood was measured and are available from Jackson and Wohl (2015) and Livers and Wohl (2016). In-stream LW volumes, also available for some of the reaches where floodplain LW measurements were taken, are from Livers and Wohl (2016).…”

Linking denitrification with ecosystem respiration in mountain streams

“…We estimated denitrification and metabolism in 11 high gradient, rocky bottomed, subalpine streams in Rocky Mountain National Park, CO and Medicine Bow National Forest, WY (Table ; Supporting Information Table A1) (Livers and Wohl ). All measurements occurred during stream baseflow after snowmelt concluded; i.e., late July and August.…”