Anne F. Lightbody

River Training and Ecological Enhancement Potential Using In-Stream Structures

The use of in-stream flow training structures for channel stabilization has become increasingly popular due to its potential cost effectiveness and ecological benefits. When properly designed and maintained, these structures help to protect the channel from erosion and lateral migration and may also provide grade control. Additionally, in-stream structures may improve fish and macroinvertebrate habitat and increase hyporheic exchange. However, a large number of these projects fail due to inadequate design guidelines. In this study, various types of in-stream sill and deflector structures are described. A literature review including case studies, journal articles, and standards developed by various agencies suggests design guidelines which are currently available but do not meet the rigorous engineering-based hydraulic design criteria necessary for success. A practitioner opinion survey of personnel from state agencies, federal agencies, and private firms indicates that these structures are being used extensively in at least 76% of the USGS physiographic provinces. In general, respondents indicated that in the areas of cost, performance, maintenance, and environmental enhancement in-stream structures are preferable to the most common alternative: riprap revetment. Information from 39 case studies suggests that successful projects involve multiple structures and are located in rivers with relatively high aspect ratios.

Ecogeomorphic feedbacks and flood loss of riparian tree seedlings in meandering channel experiments

During floods, fluvial forces interact with riparian plants to influence evolution of river morphology and floodplain plant community development. Understanding of these interactions, however, is constrained by insufficient precision and control of drivers in field settings, and insufficient realism in laboratory studies. We completed a novel set of flume experiments using woody seedlings planted on a sandbar within an outdoor meandering stream channel. We quantified effects on local sedimentation and seedling loss to scour and burial across realistic ranges of woody plant morphologies (Populus versus Tamarix species), densities (240 plants m−2 versus 24 m−2), and sediment supply (equilibrium versus deficit). Sedimentation was higher within Tamarix patches than Populus patches, reflecting Tamarixs greater crown frontal area and lower maximum crown density. Plant dislodgement occurred rarely (1% of plants) and was induced in plants with shorter roots. Complete burial was most frequent for small Tamarix that occurred at high densities. Burial risk decreased 3% for Populus and 13% for Tamarix for every centimeter increment in stem height, and was very low for plants >50 cm tall. These results suggest that Tamarix are proportionally more vulnerable than Populus when small (<20 cm tall), but that larger plants of both species are resistant to both burial and scour. Thus, plant morphological traits and development windows must be considered in addition to physical drivers when designing process-based restoration efforts on regulated rivers such as flow releases to benefit native tree species.

When do plants modify fluvial processes? Plant-hydraulic interactions under variable flow and sediment supply rates

Flow and sediment regimes shape alluvial river channels; yet the influence of these abiotic drivers can be strongly mediated by biotic factors such as the size and density of riparian vegetation. We present results from an experiment designed to identify when plants control fluvial processes and to investigate the sensitivity of fluvial processes to changes in plant characteristics versus changes in flow rate or sediment supply. Live seedlings of two species with distinct morphologies, tamarisk (Tamarix spp.) and cottonwood (Populus fremontii), were placed in different configurations in a mobile sand-bed flume. We measured the hydraulic and sediment flux responses of the channel at different flow rates and sediment supply conditions representing equilibrium (sediment supply = transport rate) and deficit (sediment supply < transport rate). We found that the hydraulic and sediment flux responses during sediment equilibrium represented a balance between abiotic and biotic factors and was sensitive to increasing flow rates and plant species and configuration. Species-specific traits controlled the hydraulic response: compared to cottonwood, which has a more tree-like morphology, the shrubby morphology of tamarisk resulted in less pronation and greater reductions in near-bed velocities, Reynolds stress, and sediment flux rates. Under sediment-deficit conditions, on the other hand, abiotic factors dampened the effect of variations in plant characteristics on the hydraulic response. We identified scenarios for which the highest stem-density patch, independent of abiotic factors, dominated the fluvial response. These results provide insight into how and when plants influence fluvial processes in natural systems.

Transport in aquatic canopies

This is the first book that reviews problems in different fluid mechanics disciplines that led to the concept of canopy, or penetrable roughness.

Fluvial sediment supply and pioneer woody seedlings as a control on bar‐surface topography

Plants influence river channel topography, but our understanding of the interaction among plants, flow, and sediment is limited, especially when sediment supply is variable. Using laboratory experiments in a recirculating flume with live seedlings in a mobile sand bed, we demonstrate how varying the balance between sediment supply and transport capacity shifts the relationship between plants and bar-surface topography. Each experimental trial contrasted two sediment conditions, in which initially supply was maintained in equilibrium with transport via sediment recirculation, followed by sediment deficit, in which transport capacity exceeded supply, which was set to zero. For both sediment balances, the topographic response was sensitive to plant size, with larger plants inducing greater aggradation relative to a baseline condition. During sediment equilibrium, the positive relationship between plant size and topographic change also depended on species morphology (multi-stemmed shrubs versus single-stemmed plants). Plant morphology effects disappeared when the sediment balance shifted to a deficit, but the presence of plants had a greater impact on the magnitude of change compared to the topographic response under sediment equilibrium. Our results suggest that the interactions among sediment supply, plants, and topography may be strongest on rivers with a balance in sediment supply and transport capacity. Because of the large variability in fluvial sediment supply resulting from natural and anthropogenic influences, these interactions will differ spatially (e.g. longitudinally through a watershed) and at different temporal scales, from single flood events to longer time periods. This article is protected by copyright. All rights reserved.

Macroinvertebrate grazers, current velocity, and bedload transport rate influence periphytic accrual in a field-scale experimental stream

Periphyton plays an important role in stream ecology, and can be sensitive to macroinvertebrate grazers, near-bed current velocity, and bedload abrasion. We manipulated conditions to examine influences on periphytic accrual in the St. Anthony Falls Laboratory Outdoor StreamLab in Minneapolis, MN, USA. Macroinvertebrate grazers were excluded from 27 of 65 clay tiles using electric pulses. We examined periphytic biomass accrual as a function of grazer presence, sampling run, and near-bed current velocity using ANCOVA. We found significant temporal differences between sampling runs but no significant effect of grazer presence. Along with a strong association between bedload transport rates and mean periphytic biomass, our results suggest that grazers are relatively unimportant in stream systems with high levels of physical disturbance from floods and associated sand bedload. However, the interaction between grazer presence and velocity was marginally significant. Regression analyses showed no relation between velocity and periphyton in the absence of grazers but a negative relation when grazers were present, suggesting that mechanical dislodgement of periphyton by grazers may increase with velocity. We conclude that grazers can have subtle effects on periphyton, particularly in streams with high bedload transport rates.

Effects of Six-Hour Suspended Sediment Treatments on White Sucker (Catostomus commersoni) and Smallmouth Bass (Micropterus dolomieu) in an Artificial Stream

ABSTRACT To examine effects of short-term exposure to two types of sediment on health of warmwater fishes, a controlled experiment was conducted with Micropterus dolomieu and Catostomus commersoni. Fishes were stocked into net pens at upstream and downstream ends of an artificial stream. Following an acclimation period of five to six days, discharge was increased throughout the stream to bankfull levels, and fishes in downstream pens (only) were exposed to sediment concentrations of 200 mg/L of fine sand or 500 mg/L of agricultural soil for six hours in two independent trials. Fishes were then immediately assigned a health assessment index value based on conditions of internal and external organs. There were no significant effects on health assessment indices for either species from the experimental treatments, although the indices may have been more sensitive if fish were afforded a post-treatment latent period to allow stress-induced changes to manifest. Our data agree with existing models that describe effects of smaller sediment particles on warmwater fishes and help address the lack of information on effects of larger, coarser suspended sediment.