THE DISCHARGE OF SEDIMENT IN CHANNELIZED ALLUVIAL STREAMS1

ABSTRACT: Approximately 400 million cubic feet of channel sediments have been delivered to the Mississippi River from the Obion-Forked Deer River system in the last 20 years. The discharge of sediment from these channelized networks in West Tennessee varies systematically with the stage of channel evolution. Variations in yields over time reflect the shifting dominance of fluvial and mass-wasting processes as the networks adjust to lower energy conditions. Maximum bed-material discharges occur during the initial phases of degradation (Stage III). In contrast, yields of suspended-sediment peak during the threshold stage (Stage 1V: large-scale mass wasting) as sediments are delivered from main-channel banks and tributary beds. Suspended-sediment yields then decrease as aggradation (Stage V) becomes the dominant trend in the main channels, but remains relatively high through restabiliza-tion (Stage VI) because of continued degradation and widening in the tributaries. Bed-material discharges decrease from the degradation stage (III) to Stage V, and increase again during restabiliza-tion (Stage VI) because secondary aggradation increases gradients and incipient meandering serves to rework bed sediments. This secondary maxima in bed-material discharge is analogous to those described previously as complex, or oscillatory, response. The trends of sediment production and transport described from these rejuvenated networks are in agreement with experimental and theoretical results of earlier investigations.     

Conditional outcomes of facilitation by a habitat-modifying subtidal bivalve

Facilitation by habitat modifiers is common in ecological communities, but the potential for temporal and spatial variations in environmental conditions to modify the outcome of these interactions and influence the strength of feedbacks is poorly understood. Suspension-feeding bivalves are important habitat modifiers that can facilitate surrounding communities by providing refuge from predation and changing boundary flows and through the production of organically enriched biodeposits. However, numerous studies have highlighted the problem of finding generalizable patterns. We tested the strength and generality of the relationship between the large suspension-feeding bivalve Atrina zelandica and surrounding macrofauna and hypothesized that facilitation by Atrina is conditional and modulated by site-specific suspended sediment concentration (SSC), which influences the quantity and quality of biodeposit production. We found temporally consistent patterns of higher rates of biodeposition and increased abundance and species richness in close proximity to Atrina under low SSC conditions. Facilitation strength decreased with increasing SSC, suggesting that the facilitation effect of Atrina is reduced and reversed along this environmental stress gradient.     

Feedbacks between bivalve density, flow, and suspended sediment concentration on patch stable states

We explore the role of biophysical feedbacks occurring at the patch scale (spatial scale of tens of meters) that influence bivalve physiological condition and affect patch stability by developing a numerical model for the pinnid bivalve, Atrina zelandica, in cohesive sediments. Simulated feedbacks involve bivalve density, flow conditions (assumed to be primarily influenced by local water depth and peak current speed), suspended sediment concentration (evaluated through a balance between background concentration, deposition, and erosion), and changes in the physiology of Atrina derived from empirical study. The model demonstrates that high bivalve density can lead to skimming flow and to a concomitant decrease in resuspension that will affect suspended sediment concentration over the patch directly feeding back on bivalve physiology. Consequently, for a given flow and background suspended sediment load, the stability of a patch directly depends on the size and density of bivalves in the patch. Although under a range of conditions patch stability is ensured independently of bivalve density, simulations clearly indicate that sudden changes in bivalve density or suspended sediment concentration can substantially affect patch structure and lead to different stable states. The model highlights the role of interactions between organisms, flow, and broader scale environmental conditions in providing a mechanistic explanation for the patchy occurrence of benthic suspension feeders.     

Serengeti wildebeest migratory patterns modeled from rainfall and new vegetation growth

We used evolutionary programming to model innate migratory pathways of wildebeest in the Serengeti Mara Ecosystem, Tanzania and Kenya. Wildebeest annually move from the southern short-grass plains of the Serengeti to the northern woodlands of the Mara. We used satellite images to create 12 average monthly and 180 10-day surfaces from 1998 to 2003 of percentage rainfall and new vegetation. The surfaces were combined in five additive and three multiplicative models, with the weightings on rainfall and new vegetation from 0% to 100%. Modeled wildebeest were first assigned random migration pathways. In simulated generations, animals best able to access rainfall and vegetation were retained, and they produced offspring with similar migratory pathways. Modeling proceeded until the best pathway was stable. In a learning phase, modeling continued with the ten-day images in the objective function. The additive model, influenced 25% by rainfall and 75% by vegetation growth, yielded the best agreement, with a multi-resolution comparison to observed densities yielding 76.8% of blocks in agreement (kappa = 0.32). Agreement was best for dry season and early wet season (kappa = 0.22-0.57), and poorest for the late wet season (0.04). The model suggests that new forage growth is a dominant correlate of wildebeest migration.