Abstract:Key processes in stream ecosystems are linked to hydraulic retention, which is the departure of stream flow from ideal 'plug flow', and reflects fluid movement through surface and hyporheic storage zones. Most existing information about hyporheic exchange is based on flume studies or field measurements in relatively steep streams with beds coarser than sand. Stream tracer studies may be used to quantify overall hydraulic retention, but disaggregation of surface and hyporheic retention remains difficult. A stream tracer approach was used to compute the rates at which stream water is exchanged with water in storage zones (total storage) in short reaches of two small, sand-bed streams under free and obstructed flow conditions. Tracer curves were fit to the one-dimensional transport with inflow storage model OTIS-P. Networks of piezometers were used to measure specific discharge between the stream and the groundwater. In the sand-bed streams studied, parameters describing total retention were in the upper 50% of data compiled from the literature, most of which represented streams with beds coarser than sand. However, hyporheic storage was an insignificant component of total hydraulic retention, representing only 0Ð01-0Ð49% of total exchange, and this fraction did not increase after installation of flow obstructions. Total retention did not vary systematically with bed material size, but increased 50-100% following flow obstruction. Removal of roughness elements, such as large wood and debris dams, is detrimental to processes dependent upon transient storage in small, sand-bed streams.

Large wood (LW) is a key component of stream habitats, and degraded streams often contain little wood relative to less-impacted ones. Habitat rehabilitation and erosion control techniques that emphasize addition of natural wood in the form of individual elements or structures are increasingly popular. However, the efficacy of wood addition, especially in physically unstable, warmwater systems is not well established. The effects of habitat rehabilitation of Little Topashaw Creek, a sinuous, sand-bed stream draining 37 km 2 in northwest Mississippi are described herein. The rehabilitation project consisted of placing 72 LW structures along eroding concave banks of a 2-km reach and planting 4000 willow cuttings in sandbars opposite or adjacent to the LW structures. Response was measured by monitoring flow, channel geometry, physical aquatic habitat and fish populations in treated and untreated reaches for 2 years before and 4 years after rehabilitation. Initially, LW structures reduced high flow velocities at concave bank toes. Progressive failure of the LW structures and renewed erosion began during the second year after rehabilitation, with only 64% of the structures and about 10% of the willow plantings surviving for 3 years. Accordingly, longterm changes in physical habitat attributable to rehabilitation were limited to an increase in LW density. Fish biomass increased in the treated reach, and species richness approximately doubled in all reaches after rehabilitation, suggesting the occurrence of some sort of stressful event prior to our study. Fish community composition shifted toward one typical of a lightly degraded reference site, but similar shifts occurred in the untreated reaches downstream, which had relatively high levels of naturally occurring LW. Large wood is a key component of sand-bed stream ecosystems, but LW addition for rehabilitation should be limited to sites with more stable beds and conditions that foster rapid woody plant colonization of sediment deposits.

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1. Allochthonous carbon is the basis of the detrital food web in low-order, warmwater stream ecosystems, and stream-bed sediments typically function as carbon reservoirs. Many of the same factors that govern carbon input and storage to streams (e.g. riparian vegetation, large wood, heterogeneous boundaries) have also been identified as key attributes of stream fish habitat.2. Effects of channel incision on sand-bed stream carbon reservoirs and indices of biological integrity (IBIs) based on fish collections were examined for four streams exhibiting a range of incisement in northern Mississippi. Observed mean C concentrations (mass percentage) ranged from 0:24 AE0:36% for a non-incised stream to only 0:01 AE 0:02% for a severely incised channel, and were not correlated with large wood (LW) density, perhaps because LW density at one site was elevated by a habitat rehabilitation project and at another site by accelerated inputs from incision-related riparian tree fall. Fish IBI was positively correlated with bed C (r ¼ 0:70; p ¼ 0:003), and IBIs for reference streams were more than 50% greater than those computed for the most severely degraded sites.3. More testing is needed to determine the efficacy of stream bed C as an indicator, but its importance to warmwater stream ecosystems, and the importance of covarying physical and hydrologic conditions seems evident.

Effects of channel incision on sand-bed stream carbon reservoirs were examined. Channel incision may deplete hyporheic zone C stores due to bed erosion, less frequent hydrologic exchanges between stream and floodplain, and paucity of riparian vegetation and large woody debris. Presented are organic C concentrations found in hyporheic sediments before and after an incised stream rehabilitation project and in three adjacent streams in northern Mississippi. The sampled streams comprise a spectrum of physical conditions corresponding to the conceptual channel evolution model (CEM). Carbon concentrations in the upper 10 cm of the bed ranged from 0.24 + 0.36% for a nonincised reference site to only 0.01 + 0.02% for aggradational incised channels. Carbon concentrations generally declined with increase in stage of the CEM, increased with increasing percent canopy over the study reach and were not directly related to large woody debris (LWD) density. These findings suggest factors linking ecological degradation to channel incision and prospective pathways for stream rehabilitation design.

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