Field delineation of geomorphic process domains along river networks in the Colorado Front Range

2013 Fall.Includes bibliographical references.Many of the conceptual models developed for river networks emphasize progressive downstream trends in morphology and processes. Such models are well-suited for larger, low-gradient rivers, but fall short in describing the extreme variability associated with headwater streams, which occupy the majority of length of stream networks, provide unique biological productivity and habitat, and can be sites of great sediment production. A more thorough understanding of the influence of local variability of process and form in headwater stream channels is required to remotely and accurately predict channel geometry characteristics for management purposes. Local variability of valley types and sediment production, or local process domains defined as glacial versus non-glacial valleys and levels of valley confinement, was evaluated for the Colorado Front Range by systematically following stream channels, categorizing them into stream type and process domain, and evaluating a number of channel geometry characteristics. The 111 reaches were then evaluated for significant differences in channel geometry among stream types and process domains, location and clustering of stream types on a slope-drainage area (S-A) plot, and downstream hydraulic geometry relationships. Statistical analyses revealed significant correlations between channel type and channel gradient, and channel type and substrate size. Although downstream hydraulic geometry relationships are well-defined using all reaches in the study area, reaches in glacial valleys display much more variability in channel geometry characteristics than reaches in fluvial valleys, as evidenced in larger ranges of channel geometry characteristics, greater difficulty in efficiently classifying stream types, less pronounced downstream hydraulic geometry relationships, and greater scatter of reaches on an S-A plot. Streams flowing through inherited terrain in glacial valleys continue to adjust to sediment and water dynamics, and level of confinement influences locations of certain stream types. Thus, local spatial variability associated with process domains at the reach scale (101-102 m) overrides progressive downstream relationships in mountain headwaters, and field calibration of relations between reach-scale channel gradient and channel characteristics is necessary to predict process and form of headwater streams in the Colorado Front Range

Instream wood loads and channel complexity in headwater southern Rocky Mountain streams under alternative states

2016 Summer.Includes bibliographical references.Small, forested streams are symbiotic with riparian processes, and thus land use changes to their riparian forests can have lasting effects on stream channel form and function. The first part of this dissertation evaluates the legacy effects of land use on stream channels of forested, subalpine streams of the Southern Rockies, with particular interest in the correlations between stream geomorphic complexity and characteristics of the adjacent riparian forest, valley geometry, and land use history. The study uses field data from the Southern Rocky Mountains of Colorado and Wyoming in streams flowing through old-growth forests (OU), younger-growth, naturally disturbed forests (YU), and forests that have undergone past land use changes (YM, management) such as logging. Field sites also have varied valley geometry (lateral confinement). Field data are used to evaluate measures of geomorphic complexity based on cross-sectional, planform, and instream wood piece and logjam variables. Significant differences in geomorphic stream complexity between OU, YU, and YM result primarily from differences in wood characteristics, which correlate strongly with pool volume and organic matter storage. Unconfined OU streams have the largest wood loads and the greatest complexity in form and function, whereas legacy effects of logging, tie-drives, and channel simplification create lowest complexity in YM streams. The second part of this dissertation proposes that the geomorphic concepts of thresholds, river metamorphosis, and complex response are the geomorphic analog to alternative states in ecology, which recognize that biotic community structure and function can exist in multiple states under the same environmental conditions. This concept is used in conjunction with field data from relatively laterally unconfined valley bottoms in the first part of this dissertation, in addition to wood data from the montane zone, to demonstrate how land use can drive streams across a threshold to induce an alternative state of significantly reduced complexity of stream form, function, and carbon storage in large wood and instream particulate organic matter. This is illustrated by threshold differences between unmanaged and managed stream segments, regardless of current forest stand age, implying that the legacy effects of past land use on riparian forest characteristics result in an alternative state of reduced stream complexity and retention. High complexity can maintain aquatic-riparian ecosystem functions through positive feedbacks, and the reduced state of managed watersheds implies an alternative ecologic state with reduced carbon storage, ecosystem productivity, and biotic diversity. The cumulative effects of reduced carbon storage in mountainous environments experiencing analogous human alteration may have large implications for global carbon budgets. Alternative states driven by land use changes likely apply to watersheds in other forested, mountain environments. Maintenance of riparian forest buffers around streams in laterally unconfined valley segments is a recommended first-order restoration technique for physical and ecological recovery

Datasets associated with ‘All Logjams Are Not Created Equal’

Data set S1: Characteristics of study reaches; Data set S2: Logjam data; both data sets provide raw data collected in the field or calculated from field-derived measurements from stream segments and logjams collected in Rocky Mountain National Park, CO in the summers of 2013 and 2014; this data is used in analyses for the associated publication.Logjams create diverse physical and ecological effects in stream channels, including at least temporary storage of water, sediment, and particulate organic matter. We hypothesize that logjams that span the entire bankfull channel width in channels < 25 m wide are more effective in storing these materials than non-channel spanning logjams. We test this hypothesis by systematically comparing characteristics of 183 logjams from 17 stream reaches in the Southern Rocky Mountains. Our dataset is novel in that it evaluates naturally occurring logjams in unaltered streams in a single study area specifically based on whether they span the stream channel. We find that channel-spanning logjams have a significantly larger number of wood pieces, longer & wider pieces, more ramp and bridge pieces, and greater logjam height and volume, both as raw data and when standardized by average channel width. Channel-spanning logjams also have significantly greater backwater pool volume and volume of particulate organic matter stored in backwater pools and in logjams. Restoration employing engineered logjams in relatively small channels currently focuses on non-spanning logjams, but could be expanded to include spanning logjams.Funding for this research was provided by NSF DEB-1145616