| Abstract |
- Numerous studies have explored how alluvial channel size and morphology are adjusted to different sediment and flow conditions, yet we still know very little about how and to what degree the flow regime controls channel form and processes. We use the term ‘channel form’ to refer to the size and shape of a stream channel, its streambed texture, planform pattern, and the presence, size, spatial configuration of bedforms, and exogenous channel elements such as woody debris. The objective of this work was to explore in more detail mounting evidence that many different aspects of the flow regime, including the range, duration, magnitude, timing, and sequencing of sediment-mobilizing flow, control stream channel form, and the processes that shape and maintain that form. In this research, we sought to identify key aspects of the flow regime that control the spatial arrangement and pattern of stream channel form, the arrangement of woody debris and its influence on flow hydraulics, and the sensitivity of sediment transport patterns to climate warming in channels with different hydrologic regimes. Studies included physical experiments, field studies, and hydrologic and sediment transport modeling.
Broadly, the purpose of the research described in Chapter 2 was to better understand how stream channels adjust to a range of flows, and to identify the timescales associated with those adjustments. Motivated by ample evidence that certain aspects of hydrograph shape, like flow duration and rate of rise and fall, control sediment transport and morphogenic processes, we designed a series of physical experiments to test whether hydrographs with different shapes led to differences in the size and spatial configuration of stream channels. Those experiments were conducted in a freely-adjustable flume that developed a self-formed, meander pattern with pool-riffle morphology. Results showed that hydrographs with slowly rising rates of rise and fall produced channels that were equivalent in size to channels generated from constant flow experiments, and had regularly spaced pool-riffle and meander patterns, while hydrographs with fast rates of rise and fall produced undersized channels with an irregular topographic structure and pool-riffle pattern. The latter finding suggests that during quickly rising hydrographs, the flow rate increased faster than the channel capacity and planform pattern could adjust. We explained differences in channel pattern organization in terms of the time available during hydrograph steps relative to the time required for adjustment, which we estimated from simple, constant flow experiments. This work points to the importance of the hydrograph as a fundamental control on channel adjustment and offers the prospect of better understanding how changes in the timing of streamflow influence channel pattern development, maintenance, and destruction.
In natural channels, stream channel form is not just a reflection of interactions among flowing water and sediment, it also reflects the added structure and roughness due to woody debris. The research objective of the Chapter 3 was to determine whether woody debris patterns reflected the underlying flow regime of the streams in which they were found, by comparing the condition and arrangement of woody debris in end-member hydrologic systems. This work was motivated by the stark contrast in the architecture of woody debris that we observed in spring-fed streams, with stable discharge regimes, and woody debris found in surface-runoff channels, that experienced more variable flows, both of which were located in the Oregon Cascades. Results indicate that differences in woody debris were so great that characteristics of woody debris may actually be a useful field indicator of overall channel flow regime. For example, the majority of wood pieces in stable, spring-fed streams were classified as immobile during a 2-5 year event in the field (72-75%), predominantly occurred as part of an open-fabric, in-situ log jam (62-84%), were covered in heavy moss accumulations or nursed vegetation (17-62%), and were often longer than the channel width and larger in size; all of which indicate a lack of fluvial transport or abrasion. In contrast, the majority of wood in runoff-dominated streams was classified as mobile (74-83%), very often had stripped back and a weathered texture (54-85%), occurred in densely-packed, transport log jams (62-84%), and was found in unstable positions resting along the channel margin and unattached to the channel boundary; all of which are indications of frequent fluvial transport and abrasion.
We interpret those results to indicate that wood rarely moves in spring-fed channels, but is mobilized during moderate flood events in the surface-runoff streams. As a result, wood in the spring-fed streams stays and decomposes in place for long periods of time (multi-decades or longer), and is often incorporated into the banks or vegetated islands. In a real sense wood literally becomes part of spring-fed channels, becoming buried in the streambed or banks, whereas wood in surface-runoff streams moves through the channel and is rarely anchored in the channel boundary. Furthermore, because wood pieces in spring-fed streams didn’t experience frequent transport and were larger in size, wood in those streams accounts for a much larger proportion of the total flow resistance. Consequently, the added flow resistance due to wood creates a hydraulic feedback loop that reinforces both wood and channel stability in those systems. Understanding how flow regimes control wood accumulations and patterns is therefore fundamental to properly interpreting the geomorphic and ecologic role of wood in streams.
In the final chapter, we modeled sediment transport patterns in spring-fed and surface-runoff channels under different climate scenarios to identify which hydrologic system will be most vulnerable to warming. We used a novel combination of hydrologic and sediment transport modeling to predict climate-driven changes in streamflow, and to estimate the extent to which those changes influenced sediment transport patterns. Although climate change is a global driver of shifts in the flow regime, few studies have investigated the effects of climate change on sediment transport regimes in the Western US, and those that have had mixed results. Median annual sediment transport volumes were predicted to increase by 1.1-4.9 times the sediment volume transported under the baseline (or no climate change) scenario in the surface-runoff channel, but were predicted to change very little (by 0.87-1.1 times the baseline transport volume) in the spring-fed channel. Changes in annual transport volume in the surface-runoff channel were driven by increases in the magnitude of moderately-sized flood events, which transported the most sediment compared to flows of any other size. In contrast, the majority of sediment in the spring-fed channel was transported by frequent baseflows, which were predicted to decline slightly in magnitude. Interestingly, slight declines in the magnitude of frequently occurring baseflows were large enough to offset increases in the magnitude of moderately-sized flood events, resulting in very little change in annual transport volume in the spring-fed channel.
We also estimated sediment transport volumes under low hydraulic roughness conditions (in the absence of woody debris), to determine whether annual transport volumes were more sensitive to climate-induced changes in the flow regime, or to reductions in hydraulic roughness following wood removal, which are likely to occur in response to more severe forest disturbances, such as wildfire. Because woody debris is a much larger source of roughness in the spring-fed channel, median annual transport volumes were predicted to increase 80-90 fold following wood removal, which was much larger than the 1.5-5.0 fold increase in transport volume predicted to occur in the surface-runoff channel following wood removal. Based on our analyses, we predict the spring-fed channel will be most vulnerable to climate change because its sediment transport volumes were more sensitive to changes in the hydraulic roughness associated with woody debris compared to the sensitivity of either study stream to climate-induced shifts in the flow regime. Furthermore, the spring-fed channel was home to valuable bull trout habitat, making its response to climate warming more critical from a management perspective. In general, climate-driven changes in sediment transport patterns, and in the hydraulic roughness due to wood, will impact streambed stability in both spring-fed and surface runoff streams, and should be considered for managing these watersheds in the future.
The results of these studies show that channel-formation and maintenance occur over multiple flows, and that the timing and distribution of those flows influence the arrangement of unique channel patterns and roughness elements, like woody debris. In particular, we’ve identified key aspects of the flow regime that control channel processes, including the time available for morphologic adjustment during a flood, the presence and frequency of wood-mobilizing flows, and the frequency of sediment-mobilizing flows relative to the threshold for mobility. By studying how stream channel form and processes integrate a variety of flows, our research helps inform how and to what degree channels are adjusted to different flow regimes and how they might respond to shifts in the flow regime. In light of pervasive changes to the natural flow regime caused by shifts in climate change, land use change, and flow regulation, a richer and more complete understanding of the hydrologic processes that shape stream channels is needed for better conservation and natural resource management.
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