H42A-01 INVITED
Detecting change in ecological controls down drainage networks
Ecosystem and food web controls of populations, trophic level biomass, and biogeochemical fluxes change down drainage networks. Field surveys, stable isotope tracers, and mensurative experiments repeated at different sites down along the upper drainages South Fork Eel River (North Coast of California) suggest that there are thresholds in drainage area where controls change on nitrogen loading from biological fixation, carbon sources to different consumer guilds in channel food webs, insect emergence, and bat tracking of this emergence. Between these thresholds, scaling relationships with channel hydraulic geometry may provide useful estimates of biomass distribution patterns of certain organisms that are relatively unaffected by consumer control, like some cyanobacteria. By investigating why controls and scaling relationships change at certain landscape positions under present environmental conditions, we hope to improve our ability to forecast how these transitions may shift with climate, land use, or biotic change, expanding, shrinking, or re-locating landscape domains under particular types of control. These steps seem necessary, but far from sufficient, for the grand challenge of linking local ecological control mechanisms to larger, basin-wide response.
H42A-02 INVITED
Aggradation at Tributary Confluences as a Control on Biodiversity in River Networks
Numerical experiments utilizing a 1-D sediment routing model demonstrate that geomorphological adjustments to altered flow and sediment regimes at tributary junctions can greatly increase physical heterogeneity in the recipient channel. This helps to explain why river confluences can behave as ‘hotspots' that exhibit elevated biodiversity. In general, biodiversity is expected to increase with physical heterogeneity if other biological factors are unchanged. In the context of understanding and managing river biodiversity at network scales, interesting issues then include (1) identifying a priori that subset of tributary junctions where large physical impacts might be expected and (2) exploring how network configuration might influence the number and spatial arrangement of such confluences. Results show that main stem aggradation is the primary driver of increased physical heterogeneity in confluence zones, and this suggests that (1) and (2) can be investigated by defining the probability of main stem aggradation at confluences. Model experiments reveal that aggradation is most sensitive to the ratios of tributary to mainstream bed load flux and bed load grain size, and is less sensitive to relative discharge. This suggests that probability statements about aggradation (and thence, perhaps, biodiversity) might be derived from estimates of the relative bed load fluxes and grain sizes at the confluences throughout a network. The potential of obtaining such estimates for real and simulated networks is considered and the importance of network magnitude and topology for network-scale patterns of confluence-zone aggradation are explored. An important implication of this work is that understanding the production, delivery and routing of sediment, not just water, is important for understanding physical and biological diversity in river networks.
H42A-03 INVITED
The importance of channel non-uniformity in riverine ecosystems
A key aspect of the hierarchical controls on riverine ecosystems that has been undervalued in both river science and restoration is the role of channel non-uniformity. As the water depth to bed grain size ratio decreases, channel non-uniformity takes over as a dominant control on hydraulic, geomorphic, and ecologic processes in rivers. As a case in point, the degree of channel degradation and habitat loss that ensues after dams are built may be directly related to maintenance of channel non-uniformity. The study reported here evaluated channel change at different spatial (submeter to km) and temporal (seasonal to interdecadal) scales in a regulated, but still dynamic, gravel-cobble river. The objectives were to (1) determine the cause of the persistence of riffles and pools even as the channel incised more than 10 m over 70 years and (2) evaluate the linkage between channel change and anadromous fish habitat in such a dynamic setting. The study area was the top 10.5 km of the Yuba River below Englebright Dam. Although this dam has blocked bedload transport into the reach for 65 years causing significant channel incision, a residual of 17.7 million m3 of sediment remains in the study reach due to historical hydraulic mining for gold. The approach used was to combine the knowledge gained at multiple scales using different methods, including historical aerial photo analysis, channel DEM analysis and differencing, 2D hydrodynamic modeling, hydraulic geometry analysis, fish observations, process-based hydrogeomorphic monitoring, and sediment transport experiments. These diverse methods revealed an array of mechanisms contributing to channel change and habitat rejuvenation, most notable including flow-convergence routing responsible for maintaining riffles and pools over a range of flows. Anadromous fish were observed to shift their pattern of habitat utilization in response to channel change in predictable ways. The primary lesson for river management is that more emphasis needs to be placed on the role of channel non-uniformity in monitoring programs as well as for river restoration.
H42A-04
Longitudinal Distribution of Wood along Headwater Streams in the Colorado Front Range
Wood loading, channel parameters, and valley parameters were surveyed in 50 contiguous stream segments each 25 m in length along 12 streams in the Colorado Front Range. Length and diameter of each piece of wood were measured, and the orientation of each piece was tallied as a ramp, sunken, bridge, or floater. These data were then used to evaluate basin- and local-scale controls on wood loading, as well as longitudinal patterns of wood distribution in forested headwater streams of the Colorado Front Range. We hypothesized that wood would be non-uniformly distributed as a result of the presence of wood jams and greater numbers of individual pieces in some segments of the stream, and that the degree of non-uniformity would vary among channels in correlation with variations in channel width, gradient, and drainage area as these parameters reflect relative capacity of a stream to transport wood introduced from the adjacent riparian zone and valley bottom. Analyses of the longitudinal uniformity of wood distribution are ongoing. Multiple regression models to evaluate controls on wood loading used either a stream-wide average or data from all 50 segments on each stream. Models that used an average for each of the 12 streams indicated that wood loading correlates most strongly with drainage area, average slope, channel width, and elevation. When all 600 stream segments were included in the multiple regression, the presence of a jam, diameter at breast height (DBH) of trees in the riparian zone, channel width, and stream gradient correlate most strongly with wood loading. Trends in sets of analyses indicate that wood loading is higher for smaller drainage areas, steeper stream gradients, narrower channels, higher elevations, the presence of a jam, and greater DBH values. Each of these correlations is physically reasonable in that smaller drainage areas and narrow channels likely have lower transport capacity, steeper streams and those with jams are hydraulically rougher and have more obstacles to wood transport, and higher elevations in the study area correspond to greater forest density.
H42A-05
Effects of Channel Type on Hyporheic Exchange in Mountain River Basins: A Process Hierarchy
Hyporheic exchange is the mixing of surface and shallow subsurface water through porous sediment surrounding a river and is driven by spatial and temporal variations in channel characteristics (streambed pressure (a function of bed topography and discharge), sediment composition, alluvial volume, and bed mobility). The significance of hyporheic exchange in linking fluvial geomorphology, groundwater, and riverine habitat for aquatic and terrestrial organisms has not been fully appreciated in the past, but has emerged in recent decades as an important component of conserving, managing, and restoring riverine ecosystems. Here, we provide a simple mathematical framework for examining the mechanics of hyporheic exchange. We propose that these mechanisms vary systematically with different channel types and associated fluvial processes that occur in mountain basins. Furthermore, the spatial distribution of hyporheic environments within mountain catchments represents a nested hierarchy of process controls; basin physiography (geology, topography, climate, and geomorphic history) influences successive scales of process domains (Montgomery 1999) and channel type (e.g., Montgomery and Buffington 1997) which, in turn, affect the local hyporheic environment (scales, rates, and magnitudes of hyporheic exchange). Consequently, a holistic view of geomorphic processes over a range of spatial and temporal scales is required for understanding hyporheic ecosystems.
H42A-06
Hierarchical controls on patterns of habitat and species diversity in river networks
Patterns of habitat heterogeneity and species diversity in river networks are constrained by a nested hierarchy of physical controls. Large-scale, long-term controls set bounds for habitat and biological expression, whereas short-term and smaller-scale processes determine conditions at a point in time. At the river basin scale, geologic and topographic controls constrain reach attributes such as channel slope and channel confinement, which in turn constrains finer scale habitat structure. Overlain on this geologic template are down-valley trends in relative sediment supply that cause a systematic shift in channel-floodplain dynamics. At the reach-scale, channel slope is a primary control on habitat types (e.g., pools, riffles, ponds) in single thread channels, but local bed load and wood supply influence local habitat diversity. In floodplain reaches, diversity of habitat types is controlled mainly by the rate of lateral channel movement and floodplain turnover, which decrease down-valley with decreasing bed load supply. These controls drive two important aspects of environmental complexity, which in turn drive biological diversity in river networks: diversity of patch ages, and diversity of patch types. Ecological theory suggests that floodplain forest communities will be most diverse in floodplain reaches with intermediate rates of floodplain turnover, and reach-level aquatic communities will be most diverse in mid-network where habitat heterogeneity is highest.
H42A-07
Reach-scale Patterns in Riverine Productivity and Diversity: Role of Tributary Junctions
Geomorphologists have long recognized that tributary streams create physical discontinuities where they flow into main stream rivers. The link between these physical changes and ecological patterns are not as well developed. Because of these physical changes, we hypothesized that tributary junctions would also be sites of high biological productivity and diversity. In 2002-2004, we examined whether tributary junctions created productivity and physical gradients, and if so, whether these gradients affected abundance and growth of invertebrates, fish and birds (2004). A large flood event in 2004 also allowed us to examine how disturbance affected these patterns. We found that tributary junctions were indeed loci for sediment and wood, as well as hotspots of biological productivity and richness. These results reinforce the notion that there are many important linkages that operate within river networks, and some of these linkages are critical for promoting ecological diversity.
H42A-08
Watershed processes, fish habitat, and salmonid distribution in the Tonsina River (Copper River watershed), Alaska
The Copper River watershed is a critical resource for northeastern Pacific salmon, with annual escapements in the millions. The Tonsina River basin, a diverse 2100-km2 tributary to the Copper River that supports important salmonid populations, offers an opportunity to integrate watershed-scale channel network data with field reconnaissance of physical processes and observed distribution of salmonid species. Our long-term goals are to characterize habitats critical to different salmonid life stages, describe the geologic context and current geologic processes that support those habitats in key channel reaches, and predict their watershed-wide distribution. The overarching motivation for these goals is resource conservation, particularly in the face of increased human activity and long-term climate change. Channel geomorphology within the Tonsina River basin reflects inherited glacial topography. Combinations of drainage areas, slopes, channel confinement, and sediment-delivery processes are unique to this environment, giving rise to channel "types" that are recognizable but that do not occur in the same positions in the channel network as in nonglaciated landscapes. We also recognize certain channel forms providing fish habitat without analog in a nonglacial landscape, notably relict floodplain potholes from once-stranded and long-melted ice blocks. Salmonid species dominated different channel types within the watershed network. Sockeye salmon juveniles were abundant in the low-gradient, turbid mainstem; Chinook juveniles were also captured in the lower mainstem, with abundant evidence of spawning farther downstream. Coho juveniles were abundant in upper, relatively large tributaries, even those channels with cobble-boulder substrates and minimal woody debris that provide habitats more commonly utilized by Chinook in low-latitude systems. More detailed field sampling also revealed that patterns of species composition and abundance appeared related to small-scale differences in physical habitat features. For example, juvenile coho salmon used interstitial spaces between unembedded cobbles and boulders but were absent from adjacent habitat with high embeddedness. Thus high delivery rates of coarse sediment sustain critical rearing habitat that would otherwise be relatively inhospitable to fish. Using Chinook salmon as a focal species, we have integrated field- and map-based analyses to predict basin- scale geomorphic and biological constraints on the distribution of suitable spawning and rearing habitat. These analyses provide rapid guidance for where focused investigations or monitoring of key habitats should occur, a particularly important outcome where watersheds are large and field logistics are challenging. The predicted extent of suitable stream habitat within the study area represents a relatively minor fraction (ca. 10 percent) of the total stream channel network, suggesting that production of salmon from the study area depends on the maintenance of quality habitat in discrete, and relatively rare, reaches.