Intergranular flow velocity through salmonid redds: sensitivity to fines infiltration from low intensity sediment transport events

Abstract

This paper presents results from a novel technique allowing continuous monitoring through multiple storm events of interstitial flow in salmonid redds. Previous studies have shown that long-term increases in fine sediment inputs into rivers can silt up spawning beds, reduce intergravel flow and threaten egg survival. Not enough is known, however, about the temporal and spatial scales of the physical processes affecting spawning habitat. The short-term sensitivity of intergravel flow through salmon nests to low-intensity sediment transport events has not been documented. Furthermore, it is unclear if the egg pocket flow vital to incubation is principally controlled by the hydraulic conductivity of the redd patch or by that, generally lower, of the ambient riffle substrate. The purpose of this study was to determine if individual runoff events could affect intergravel flow in salmon nests and to investigate the sensitivity of interstitial flow to the fines content and conductivity of the redd patch. During the summer and autumn of 2001, a new intergravel velocity sensor based on the hot wire principle made it possible to continuously monitor, over five months, interstitial velocities in artificial redds in four tributaries of the Cascapedia River, Quebec. Fifteen low and moderate intensity runoff events (up to 50% bankfull) were monitored. Data were obtained for each storm on suspended sediment transport as well as sand infiltration rates in sediment collectors emplaced in redd zones. It was found that redd interstitial velocities were reduced whenever a runoff event deposited more than 7 kg/m2 of sands in infiltration traps. In addition, redd interstitial velocities were reduced four out of the five times that the event-integrated suspended sediment dose exceeded 7 mg l−1 day (dose is defined as the area under the concentration time curve). In the study conditions, where ambient riffle sediment has relatively moderate permeability and localized groundwater upwelling is negligible, our data suggest that significant intergravel flow (0.1–0.6 mm/s) can be triggered through 2 m long redd patches, in response to the redd-scale water surface gradient and the relatively higher conductivity of the redd patch, after spawner activity. Copyright © 2005 John Wiley & Sons, Ltd.