Wood-induced stream channel complexity as a driver of transient storage
Date
2021
Authors
Marshall, Anna E., author
Wohl, Ellen, advisor
Morrison, Ryan, committee member
Rathburn, Sara, committee member
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Abstract
Rivers are naturally diverse, heterogeneous systems. This spatial heterogeneity is driven by inputs into a river corridor including water, sediment, large wood, coarse particulate organic material (CPOM), and dissolved loads, all of which interact with the river valley to create distinct forms of geomorphic complexity. Spatial heterogeneity within a river corridor drives surface exchange of water, sediment, and nutrients moving downstream and into transient storage zones. Transient storage is increased by features that enhance bed heterogeneity and surface flow separation, such as large wood. This thesis explores the role that wood-induced spatial heterogeneity has in facilitating zones of surface transport and storage. Specifically, I look at how discharge and logjam characteristics drive transient storage, as reflected in the movement of salt tracers and CPOM. Logjams alter gradients in hydraulic head and create zones of flow-separation and low velocity along channel margins, which enhance storage of organic matter and solutes. Although research has shown that a single logjam in a channel increases transient storage, limited work has been done to understand the characteristics of logjams that enhance transient storage. Scientific gaps remain in understanding whether decreasing the downstream spacing of logjams or the logjam porosity drives an increase in transient storage. Here, I designed experiments for two constructed flume systems—one with a change in downstream logjam spacing and one with a change in logjam porosity—to understand the effects of logjam characteristics on transient storage at different flows. More closely spaced logjams resulted in slower advection down the flume channel at low flows. At high flows and with changing porosities, there was no consistent effect of jam spacing or jam porosity on advective travel times. This likely points to the influence of a low Damkohler number, where high velocity and/or short reach length resulted in only a small amount of flow-path exchange with storage zones. In the future, a modified flume configuration with exaggerated scaling of discharge and porosity may be necessary to better distinguish the study effects. Surface transient storage is also a physical control on CPOM transport and deposition. The details of surface transient storage determine when and if CPOM is deposited and remobilized. Yet, no studies have looked at how CPOM transport and storage vary in relation to shorter (diurnal) as well as longer (seasonal peak flow) variations in discharge. I physically sampled CPOM moving along the bed and in-suspension at stream reaches above and below a logjam as well as a location with no logjam. I sampled CPOM masses throughout a seasonal hydrograph and on a 24-hour diurnal timespan to examine how transport and deposition changed with flow. The majority of CPOM was transported in suspension following a clockwise hysteresis along the seasonal hydrograph. CPOM stored in the channel and overbank areas is more likely to be mobilized as stage rises and snowmelt runoff enters the channel, whereas the supply of CPOM is depleted as the snowmelt hydrograph continues. A similar hysteresis in CPOM transport did not occur during 24-hour diurnal fluctuations in discharge except at peak CPOM movement. CPOM peaked before discharge peaked on both the diurnal and seasonal hydrographs. CPOM transport in a logjam backwater occurred at a significantly lower rate than in reaches without logjams, suggesting logjams provide storage zones for CPOM. Understanding the characteristics and processes of wood-induced spatial heterogeneity that facilitate zones of surface transport and storage has important management implications. Large wood and hyporheic restoration are increasingly used to enhance ecosystem services and functions in rivers, yet the specific characteristics of logjams remain poorly defined. Considering that CPOM is a primary energy source in the food webs of shaded forest streams, management designed to foster the sustainability of stream ecosystems can benefit from maintaining or creating features that enhance CPOM retention. Furthermore, designing logjams to restore a river reach with the goal of improving hydrologic function will provide greater value if the design incorporates reach-scale logjam characteristics that enhance transient storage.
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Subject
hyporheic exchange
organic matter
transient storage
large wood
fluvial geomorphology
rivers