Fine particle retention within stream storage areas at base flow and in response to a storm event

Abstract

Fine particles (1–100 µm), including particulate organic carbon (POC) and fine sediment, influence stream ecological functioning because they may contain or have a high affinity to sorb nitrogen and phosphorus. These particles are immobilized within stream storage areas, especially hyporheic sediments and benthic biofilms. However, fine particles are also known to remobilize under all flow conditions. This combination of downstream transport and transient retention, influenced by stream geomorphology, controls the distribution of residence times over which fine particles influence stream ecosystems. The main objective of this study was to quantify immobilization and remobilization rates of fine particles in a third‐order sand‐and‐gravel bed stream (Difficult Run, Virginia, USA) within different geomorphic units of the stream (i.e., pool, lateral cavity, and thalweg). During our field injection experiment, a thunderstorm‐driven spate allowed us to observe fine particle dynamics during both base flow and in response to increased flow. Solute and fine particles were measured within stream surface waters, pore waters, sediment cores, and biofilms on cobbles. Measurements were taken at four different subsurface locations with varying geomorphology and at multiple depths. Approximately 68% of injected fine particles were retained during base flow until the onset of the spate. Retention was evident even after the spate, with 15.4% of the fine particles deposited during base flow still retained within benthic biofilms on cobbles and 14.9% within hyporheic sediment after the spate. Thus, through the combination of short‐term remobilization and long‐term retention, fine particles can serve as sources of carbon and nutrients to downstream ecosystems over a range of time scales.

Less Fine Particle Retention in a Restored Versus Unrestored Urban Stream: Balance Between Hyporheic Exchange, Resuspension, and Immobilization

“…Although previous studies have supported reworking of the bed during storm flow (Drummond et al, ; Gartner et al, ; Harvey et al, ), the results presented here indicate rapid reworking of fine particles within the bed under baseflow conditions. Furthermore, in the absence of whole‐bed mobilizing events, particle deposition will eventually become self‐limiting through clogging of interstices (Hartwig & Borchardt, ; Packman & MacKay, ).…”

“…These structures may favor deposition and retention of fine particles by reducing near‐bank flow velocity; however, our finding that net particle immobilization was lower in the restored stream suggests that the unrestored stream is more efficient in retaining fine particles. It is likely that the greater channel complexity of the unrestored stream, with its emergent bars and riffles, woody debris, and lateral channel variability, contributes to greater net particle immobilization in the unrestored stream (Drummond et al, ). Our findings are consistent with a previous observation that the unrestored stream has a greater abundance of fine particles within surface storage zones than the restored stream (Larsen & Harvey, ).…”

“…As part of a larger set of urban stream studies (Drummond et al, ; Knapp et al, ; Larsen & Harvey, ), we compared the results from the tracer injection experiment presented here (Accotink Creek, restored stream) with those obtained from a previously published injection experiment in Difficult Run, a nearby unrestored stream in an adjacent watershed with similar land‐use and impervious surface characteristics. While Difficult Run is devoid of cross vanes or bank stabilization features, its watershed does contain distributed stormwater best management practices in the form of detention basins.…”

“…Local hyporheic exchange of solutes and particles (defined as the hyporheic piston velocity and local fine particle immobilization velocity, respectively) were estimated following the methods described in Harvey et al () and Drummond et al (). The piston velocity of stream water into the hyporheic zone, V ′ h , was estimated directly from field data using measurements of streambed sediment porosity ( θ ), the advective tracer traveltime ( t h ) to the depth of the hyporheic measurement depth ( d h ), and the fraction of surface water in hyporheic flow paths at the measurement depth ( f sw ): …”

Improving Predictions of Fine Particle Immobilization in Streams

“…Both organic and mineral fine particles provide a colonizable surface area for bacteria (Mendoza-Lera et al, 2016) and, therefore, strongly influence pathogen transmission in streams (Bradford et al, 2013;Drummond et al, 2018). Although fine particles have Shields and Rouse numbers typical of washload, numerous prior studies have found that fine particle immobilization process dynamics are much more complex with long-term retention, both in surface waters and storage areas (e.g., Drummond et al, 2017;Newbold et al, 2005;Cushing et al, 1993;Harvey et al, 2012;Karwan & Saiers, 2012), indicating a need to improve predictive capabilities to forecast responses to perturbations and support risk assessments.…”