Ellen Wohl - Academia.edu (original) (raw)
Papers by Ellen Wohl
... pond sediment by determining the rates and types of sedimentation since the middle ... 1.3 Ob... more ... pond sediment by determining the rates and types of sedimentation since the middle ... 1.3 Objectives Sediments deposited in valley bottoms record watershed-scale landscape processes ... photographs of Beaver Meadows and Moraine Park from the late 1930s to 2001. In Beaver ...
ABSTRACT We surveyed wood characteristics along four headwater channel segments of the Colorado F... more ABSTRACT We surveyed wood characteristics along four headwater channel segments of the Colorado Front Range with drainage areas of 8–82 km2. Stream lengths surveyed range from 3025 to 8980 m and together include a total of 15,706 pieces of wood. Time since last disturbance in the form of a stand-killing fire varied from 31 to > 500 years. Individual pieces of wood were highly aggregated at length scales of 1 to 150 m. Trends among jams were more weakly developed, but jams tended to be more segregated at lengths < 10 m, slightly more aggregated at lengths ~ 100–300 m, and to have diverging patterns at lengths > 300 m, with jams along individual channels being aggregated, segregated, or random. Multiple linear regressions failed to produce highly predictive models to explain the response variables of wood load, piece dimensions, or characteristics of jams other than jam volume (which correlated with drainage area and wood load); but examination of downstream patterns suggests that local valley and channel geometry (valley-bottom width, gradient, and sequence of longitudinal channel changes) exert a stronger influence on patterns of wood distribution than either time since last forest disturbance or progressive downstream trends associated with increasing drainage area. The longitudinal sequences of wood recruitment sources, forest stand age, and channel geometry together exert an important control on reach-scale wood load and aggregation. Wood loads in streams draining old-growth (> 250 years) forests of the Colorado Front Range are low compared to old-growth sites in other regions of the world, which we interpret to reflect decreased retention of wood recruited to the streams.
This study used three cross-sections along a subalpine channel to examine associations between fl... more This study used three cross-sections along a subalpine channel to examine associations between flow and sand and gravel bedload transport in a cobble-bed channel. At each section, velocity was measured at 0.2, 0.4, 0.6, and 0.8 of flow depth for ten increments across the section. At each increment, velocity measurements were immediately followed by 10-min bedload samples with a 7.6 crn Helley-Smith sampler. Linear regressions between bedload and flow variables suggest that grain shear stress, stream power, near-bed velocity, average velocity, and the Froude number are the most relevant indicators of bedload transport, particularly when regressed against the D~ or Ds4 of the bedload grain-size distribution. No significant relationships, however, were constant at all three cross-sections. Inconsistency in relationships between bedload discharge and hydraulic variables is at least in part the result of the extreme temporal and spatial variability present in East St. Louis Creek. Photographs and observations made throughout the field season suggest the importance of random events, such as tree-fall and log-jam formation, in regulating bedload discharge. The data from East St. Louis Creek suggest equal mobility for sand and gravel transport.
We use a large and diverse data set from mountain streams around the world to explore relationshi... more We use a large and diverse data set from mountain streams around the world to explore relationships between reach-scale channel morphology and control variables. The data set includes 177 step-pool reaches, 44 plane-bed reaches, and 114 pool-riffle reaches from the western United States, Panama, and New Zealand. We performed several iterations of stepwise discriminant analysis on these data. A three-variable
We investigated the effects of morphologic position and discharge on flow structure in a steep (0... more We investigated the effects of morphologic position and discharge on flow structure in a steep (0.10m/m) mountain channel by collecting three-dimensional measurements of time-averaged and turbulent velocity components with a SonTek FlowTracker Handheld ADV (acoustic Doppler velocimeter) on a 30-m reach of a step-pool channel in the Colorado Rockies. Velocity profiles were measured at morphologic positions characteristic of steep channels
The basic patterns and processes of steep channels remain poorly known relative to lower-gradient... more The basic patterns and processes of steep channels remain poorly known relative to lower-gradient channels. In this analysis, characteristics of step-pool, plane-bed, and pool-riffle channels are examined using a data set of 335 channel reaches from the western United States, Nepal, New Zealand, and Panama. We analyzed differences among the three channel types with respect to hydraulics, channel geometry, boundary roughness, and bedforms. Step-pool channels have significantly steeper gradients, coarser substrate, higher values of shear stress and stream power for a given discharge, and larger ratios of bedform amplitude/wavelength (H/L). Pool-riffle channels have greater width/depth ratios and relative grain submergence (R/D84) than the other channel types. Plane-bed channels tend to have intermediate values for most variables examined. Relative form submergence (R/H) is statistically similar for step-pool and pool-riffle channels. Despite the lesser relative grain submergence and greater bedform amplitude of step-pool channels, mean values of Darcy–Weisbach friction factor do not change in response to changes in relative grain submergence. These patterns suggest that adjustments along mountain streams effectively maximize resistance to flow and minimize downstream variability in resistance among the different channel types.
A dataset of 21 study reaches in the Porter and Kowai rivers (eastern side of the South Island), ... more A dataset of 21 study reaches in the Porter and Kowai rivers (eastern side of the South Island), and 13 study reaches in Camp Creek and adjacent catchments (western side of the South Island) was used to examine downstream hydraulic geometry of mountain streams in New Zealand. Streams in the eastern and western regions both exhibit well-developed downstream hydraulic geometry, as indicated by strong correlations between channel top width, bankfull depth, mean velocity, and bankfull discharge. Exponents for the hydraulic geometry relations are similar to average values for rivers worldwide. Factors such as colluvial sediment input to the channels, colluvial processes along the channels, tectonic uplift, and discontinuous bedrock exposure along the channels might be expected to complicate adjustment of channel geometry to downstream increases in discharge. The presence of well-developed downstream hydraulic geometry relations despite these complicating factors is interpreted to indicate that the ratio of hydraulic driving forces to substrate resisting forces is sufficiently large to permit channel adjustment to relatively frequent discharges. q 2004 Published by Elsevier B.V.
... Mediterranean Desertification: A Mosaic of Processes and Responses . Edited by Nichola Geeson... more ... Mediterranean Desertification: A Mosaic of Processes and Responses . Edited by Nichola Geeson, Jane Brandt and John B. Thornes Chichester : John Wiley &amp; Sons , 2002 , 440 pp. ... Ian Foster, Coventry University. Tree Cultures: The Place of Trees and Trees in Their Place . ...
A reservoir sediment release on the North Fork Poudre River supplied ~7000 m3 of silt- to pebble-... more A reservoir sediment release on the North Fork Poudre River supplied ~7000 m3 of silt- to pebble-sized sediment to an originally boulder bed channel. Deposition along the 12 km of channel downstream from the reservoir occurred primarily in pools. During the subsequent snowmelt hydrograph, sediment was progressively scoured from the upstream and then the downstream pools. Initial sediment reworking in the pools created a deep, narrow thalweg scoured to the original pool bed, with additional sediment deposition in lateral eddies. Continued reworking reduced but did not completely remove these eddy deposits. The channel became supply-limited with respect to finer grain-sized fractions (clay to medium sand) first at upstream and then at downstream sites and eventually became supply-limited with respect to coarser grain-sized fractions (coarse sand to pebbles). Bedload transport rates at a site were strongly linked to the depletion of sediment stored in upstream pools. Magnitude, duration, and sequence of flows were all important controls on bedload transport and return of the channel to its prerelease state.
Nature Climate Change, 2012
NATURE CLIMATE CHANGE | VOL 2 | SEPTEMBER 2012 | www.nature.com/natureclimatechange 655 F resh wa... more NATURE CLIMATE CHANGE | VOL 2 | SEPTEMBER 2012 | www.nature.com/natureclimatechange 655 F resh water is under pressure in the humid tropics from population growth, land use and climate change, all of which are influenced by humans. These pressures are likely to have profound consequences. Here we present a research vision for the humid tropics as an outcome of a community workshop held in Hawaii in March 2011. This report includes various perspectives from the international scientific community and examines the key role that hydrology plays in the functioning of the humid tropics in this age of human impact on all facets of the natural environment. Processes that operate within the hydrological cycle are expected to accelerate as temperatures rise and the capacity of the air to carry moisture increases. Understanding of key interactions is limited geographically and relies heavily on model-based scenarios rather than observations. Better understanding of interactions within the hydrological cycle is needed for the hydrological community to actively address adaptation and mitigation strategies for anthropogenic and climate changes. Of particular concern are interactions among atmospheric moisture fluxes and vegetation, soil water and energy balances, near-surface and subsurface processes (including groundwater resources), and stream flow and the transport of solutes and sediments ( ).
Water Resources Research, 2009
We used a mobile bed flume with scaled grain size distribution, channel geometry, and flow to exa... more We used a mobile bed flume with scaled grain size distribution, channel geometry, and flow to examine morphology and hydraulics of stepped channels. We hypothesized that (1) step geometry and flow resistance differs significantly as a function of the range of grain sizes present, (2) a transition from nappe to skimming flow occurs in stepped channels with mobile beds for
... pond sediment by determining the rates and types of sedimentation since the middle ... 1.3 Ob... more ... pond sediment by determining the rates and types of sedimentation since the middle ... 1.3 Objectives Sediments deposited in valley bottoms record watershed-scale landscape processes ... photographs of Beaver Meadows and Moraine Park from the late 1930s to 2001. In Beaver ...
ABSTRACT We surveyed wood characteristics along four headwater channel segments of the Colorado F... more ABSTRACT We surveyed wood characteristics along four headwater channel segments of the Colorado Front Range with drainage areas of 8–82 km2. Stream lengths surveyed range from 3025 to 8980 m and together include a total of 15,706 pieces of wood. Time since last disturbance in the form of a stand-killing fire varied from 31 to > 500 years. Individual pieces of wood were highly aggregated at length scales of 1 to 150 m. Trends among jams were more weakly developed, but jams tended to be more segregated at lengths < 10 m, slightly more aggregated at lengths ~ 100–300 m, and to have diverging patterns at lengths > 300 m, with jams along individual channels being aggregated, segregated, or random. Multiple linear regressions failed to produce highly predictive models to explain the response variables of wood load, piece dimensions, or characteristics of jams other than jam volume (which correlated with drainage area and wood load); but examination of downstream patterns suggests that local valley and channel geometry (valley-bottom width, gradient, and sequence of longitudinal channel changes) exert a stronger influence on patterns of wood distribution than either time since last forest disturbance or progressive downstream trends associated with increasing drainage area. The longitudinal sequences of wood recruitment sources, forest stand age, and channel geometry together exert an important control on reach-scale wood load and aggregation. Wood loads in streams draining old-growth (> 250 years) forests of the Colorado Front Range are low compared to old-growth sites in other regions of the world, which we interpret to reflect decreased retention of wood recruited to the streams.
This study used three cross-sections along a subalpine channel to examine associations between fl... more This study used three cross-sections along a subalpine channel to examine associations between flow and sand and gravel bedload transport in a cobble-bed channel. At each section, velocity was measured at 0.2, 0.4, 0.6, and 0.8 of flow depth for ten increments across the section. At each increment, velocity measurements were immediately followed by 10-min bedload samples with a 7.6 crn Helley-Smith sampler. Linear regressions between bedload and flow variables suggest that grain shear stress, stream power, near-bed velocity, average velocity, and the Froude number are the most relevant indicators of bedload transport, particularly when regressed against the D~ or Ds4 of the bedload grain-size distribution. No significant relationships, however, were constant at all three cross-sections. Inconsistency in relationships between bedload discharge and hydraulic variables is at least in part the result of the extreme temporal and spatial variability present in East St. Louis Creek. Photographs and observations made throughout the field season suggest the importance of random events, such as tree-fall and log-jam formation, in regulating bedload discharge. The data from East St. Louis Creek suggest equal mobility for sand and gravel transport.
We use a large and diverse data set from mountain streams around the world to explore relationshi... more We use a large and diverse data set from mountain streams around the world to explore relationships between reach-scale channel morphology and control variables. The data set includes 177 step-pool reaches, 44 plane-bed reaches, and 114 pool-riffle reaches from the western United States, Panama, and New Zealand. We performed several iterations of stepwise discriminant analysis on these data. A three-variable
We investigated the effects of morphologic position and discharge on flow structure in a steep (0... more We investigated the effects of morphologic position and discharge on flow structure in a steep (0.10m/m) mountain channel by collecting three-dimensional measurements of time-averaged and turbulent velocity components with a SonTek FlowTracker Handheld ADV (acoustic Doppler velocimeter) on a 30-m reach of a step-pool channel in the Colorado Rockies. Velocity profiles were measured at morphologic positions characteristic of steep channels
The basic patterns and processes of steep channels remain poorly known relative to lower-gradient... more The basic patterns and processes of steep channels remain poorly known relative to lower-gradient channels. In this analysis, characteristics of step-pool, plane-bed, and pool-riffle channels are examined using a data set of 335 channel reaches from the western United States, Nepal, New Zealand, and Panama. We analyzed differences among the three channel types with respect to hydraulics, channel geometry, boundary roughness, and bedforms. Step-pool channels have significantly steeper gradients, coarser substrate, higher values of shear stress and stream power for a given discharge, and larger ratios of bedform amplitude/wavelength (H/L). Pool-riffle channels have greater width/depth ratios and relative grain submergence (R/D84) than the other channel types. Plane-bed channels tend to have intermediate values for most variables examined. Relative form submergence (R/H) is statistically similar for step-pool and pool-riffle channels. Despite the lesser relative grain submergence and greater bedform amplitude of step-pool channels, mean values of Darcy–Weisbach friction factor do not change in response to changes in relative grain submergence. These patterns suggest that adjustments along mountain streams effectively maximize resistance to flow and minimize downstream variability in resistance among the different channel types.
A dataset of 21 study reaches in the Porter and Kowai rivers (eastern side of the South Island), ... more A dataset of 21 study reaches in the Porter and Kowai rivers (eastern side of the South Island), and 13 study reaches in Camp Creek and adjacent catchments (western side of the South Island) was used to examine downstream hydraulic geometry of mountain streams in New Zealand. Streams in the eastern and western regions both exhibit well-developed downstream hydraulic geometry, as indicated by strong correlations between channel top width, bankfull depth, mean velocity, and bankfull discharge. Exponents for the hydraulic geometry relations are similar to average values for rivers worldwide. Factors such as colluvial sediment input to the channels, colluvial processes along the channels, tectonic uplift, and discontinuous bedrock exposure along the channels might be expected to complicate adjustment of channel geometry to downstream increases in discharge. The presence of well-developed downstream hydraulic geometry relations despite these complicating factors is interpreted to indicate that the ratio of hydraulic driving forces to substrate resisting forces is sufficiently large to permit channel adjustment to relatively frequent discharges. q 2004 Published by Elsevier B.V.
... Mediterranean Desertification: A Mosaic of Processes and Responses . Edited by Nichola Geeson... more ... Mediterranean Desertification: A Mosaic of Processes and Responses . Edited by Nichola Geeson, Jane Brandt and John B. Thornes Chichester : John Wiley &amp; Sons , 2002 , 440 pp. ... Ian Foster, Coventry University. Tree Cultures: The Place of Trees and Trees in Their Place . ...
A reservoir sediment release on the North Fork Poudre River supplied ~7000 m3 of silt- to pebble-... more A reservoir sediment release on the North Fork Poudre River supplied ~7000 m3 of silt- to pebble-sized sediment to an originally boulder bed channel. Deposition along the 12 km of channel downstream from the reservoir occurred primarily in pools. During the subsequent snowmelt hydrograph, sediment was progressively scoured from the upstream and then the downstream pools. Initial sediment reworking in the pools created a deep, narrow thalweg scoured to the original pool bed, with additional sediment deposition in lateral eddies. Continued reworking reduced but did not completely remove these eddy deposits. The channel became supply-limited with respect to finer grain-sized fractions (clay to medium sand) first at upstream and then at downstream sites and eventually became supply-limited with respect to coarser grain-sized fractions (coarse sand to pebbles). Bedload transport rates at a site were strongly linked to the depletion of sediment stored in upstream pools. Magnitude, duration, and sequence of flows were all important controls on bedload transport and return of the channel to its prerelease state.
Nature Climate Change, 2012
NATURE CLIMATE CHANGE | VOL 2 | SEPTEMBER 2012 | www.nature.com/natureclimatechange 655 F resh wa... more NATURE CLIMATE CHANGE | VOL 2 | SEPTEMBER 2012 | www.nature.com/natureclimatechange 655 F resh water is under pressure in the humid tropics from population growth, land use and climate change, all of which are influenced by humans. These pressures are likely to have profound consequences. Here we present a research vision for the humid tropics as an outcome of a community workshop held in Hawaii in March 2011. This report includes various perspectives from the international scientific community and examines the key role that hydrology plays in the functioning of the humid tropics in this age of human impact on all facets of the natural environment. Processes that operate within the hydrological cycle are expected to accelerate as temperatures rise and the capacity of the air to carry moisture increases. Understanding of key interactions is limited geographically and relies heavily on model-based scenarios rather than observations. Better understanding of interactions within the hydrological cycle is needed for the hydrological community to actively address adaptation and mitigation strategies for anthropogenic and climate changes. Of particular concern are interactions among atmospheric moisture fluxes and vegetation, soil water and energy balances, near-surface and subsurface processes (including groundwater resources), and stream flow and the transport of solutes and sediments ( ).
Water Resources Research, 2009
We used a mobile bed flume with scaled grain size distribution, channel geometry, and flow to exa... more We used a mobile bed flume with scaled grain size distribution, channel geometry, and flow to examine morphology and hydraulics of stepped channels. We hypothesized that (1) step geometry and flow resistance differs significantly as a function of the range of grain sizes present, (2) a transition from nappe to skimming flow occurs in stepped channels with mobile beds for