Interactions Between Physical and Biological Processes in Riverine Landscapes III: Spatial Heterogeneity and Scales of Interactions B
Presiding: J M Buffington, USDA Forest Service; C V Baxter, Colorado State University; A E Rosenberger, University of Idaho and USDA Forest Service
B42A-01 INVITED 10:30h
Scales of Stream Disturbance Patterns and Population Structure in Bull Trout
Ecological theory proposes that the geometry and dynamics of suitable habitats are important predictors for the persistence of a population or metapopulation. A key finding supporting a metapopulation-like conceptualization of extinction and colonization in fragmented salmonid populations is that individuals of particular species are more likely to be absent from small patches of suitable habitat than from large patches. Extinctions from small patches occur for a few reasons that can be roughly classed as physical catastrophic causes (e.g. a major channel reorganizing event) or as biological small-population effects related to genetics and demographics. One important question with implications for land management is how strong of a role physical disturbances play in determining presence and absence of a species within a patch. If disturbance plays a key role in structuring populations, then we would expect to observe two patterns. First the spatial scale of disturbance should be on the order of the size of patches that are commonly unoccupied. The second prediction would be that the spatial scale of genetic variability should show strong gene flow at scales smaller than the scale of disturbance. We examined aerial photography over the last 40 years in central Idaho to measure the scale of stream disturbance patches. The scale of stream disturbances was on the same order of magnitude as the divide between commonly occupied and unoccupied patches, and was smaller than scales of high gene flow among bull trout populations, failing to reject the possibility that disturbance plays an important role in structuring populations in this basin. Management for persistence of the species must consider both the likelihood of decreased habitat patch sizes and increased disturbance scales with global warming. Knowing disturbance patch scales and how disturbance operates to structure populations can be used to consider a disturbance based paradigm for management of sensitive stocks using a minimum dynamic area approach in wildland systems to replace the loading based approach to cumulative watershed effects as exemplified by clean water act regulations.
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B42A-02 10:45h
Assessing the Relative Roles of Environmental Factors on Population Scaling: An Example with Chinook Salmon
To effectively manage and conserve a growing list of dwindling species requires understanding the geographic scales at which populations are structured and discerning the environmental characteristics that affect population structure. Spatially referenced datasets, fine-scale genetic studies, and spatial autocorrelation analyses are now yielding insights to key population parameters such as dispersal and genetic neighborhood size, but determination of environmental correlates has lagged behind for many species. We present a simple framework for ascertaining the importance of environmental features on these population attributes. The basis of the approach consists of comparing spatial signatures for population parameters to signatures derived for a host of environmental features across a gradient of landscape types. The expectation was that biological attributes would track changes in the most important environmetal characterists. We illustrate the approach by constructing spatial correlograms for genetic and demographic data from Chinook salmon and comparing these to correlograms derived for several stream network features. Comparisons were made between two landscapes that differed with regards to disturbance and patchiness of spawning environments. Shortcomings to the proposed framework exist, but it does provide a first step towards identifying environmental factors which constrain important population parameters.
B42A-03 11:00h
Tributary Sediment Yield, Water Yield and Sediment Composition as Controls of Habitat Heterogeneity in River Networks
There is growing evidence that the delivery of water and sediment from tributaries can, via changes in habitat, elicit biological responses in the recipient channel. In particular, greater habitat heterogeneity near confluences may support biodiversity "hotspots" of broad, ecosystem significance. However, our understanding of what controls tributary impacts is very limited. This hinders effective consideration of confluences in river management practice; a problem that is pressing because of recognition that lotic ecosystems involve bio-physical interactions across networks, rather than along simple drainage lines. To date, field data have been used to specify empirical rules relating the probability of a tributary's impact to basin properties such as relative area. This approach has the advantage that it is easily applied at landscape scales, but because basin characteristics are used as gross surrogates for sediment flux, water flux, and sediment composition, little has been learned about the operation and interaction of these primary controls. We therefore used a 1-D sediment routing model with multiple grain-size fractions to investigate mainstream responses to inputs of water and sediment at confluences. Simulations were performed for different initial conditions, using many combinations of values of three independent parameters: the ratios of tributary to mainstream water flux (QR), sediment flux (FR) and characteristic bed load grain size (DR). A primary distinction can be made between confluences that exhibit aggradation (DR > 2) and those that exhibit degradation (DR < 1 or FR = 0), but in the real world post-confluence degradation is rare and we therefore focus here on aggrading tributaries. We describe the impact of QR, FR and DR on the longitudinal spatial heterogeneity of three indicative characteristics of physical habitat: mainstream grain size, percentage sand and Froude number. To apply our results at broader scales requires specification of relations between QR, FR, DR and easily measured network or basin parameters. This issue is considered, and tentative inferences about the network-scale implications of our results are made.
B42A-04 11:15h
The Influence of Modern and Holocene Tributary Sedimentation Events on Main-Stem Channel Heterogeneity, South Fork Payette River, Idaho
Sediment-charged floods and debris flows from tributaries are a major source of sediment to the main channel of the South Fork Payette River. A single debris-flow event in 1997 deposited ~ 11,000 m3 of sediment in the main channel, but a lack of cobble-to-boulder sized clasts resulted in little change in mainstem channel morphology. Conversely, large boulders in other tributary debris flows produced persistent major channel rearrangement during the same storm. Analysis and radiocarbon dating of alluvial fan stratigraphy from 32 tributary fans reveals a record of sedimentation events spanning the last ca. 8000 years. Estimated event magnitudes based on deposit thickness indicate that magnitudes vary greatly over time, likely with Holocene climate changes. Over the last ca. 4000 years, small sedimentation events are almost twice as common as large events, but large events account for 66% of the total dated fan thickness. Only 9 large fire-induced debris flows emplaced ~25% of the total dated fan thickness during the Medieval Climatic Anomaly (ca. 1000-800 cal yr BP), a time of widespread and severe western US droughts. Since rare large sedimentation events strongly affect sediment yields and channel heterogeneity, climatic control of these events has major implications for riverine habitat and populations.
B42A-05 11:30h
Slope-Area Controls on the Expression of Reach-Scale Channel Morphology, Debris Flow Runout, and the Spatial Distribution of Salmonids in Steep Mountain Streams
Steepness and concavity indexes derived from the power function relationship between drainage area and channel slope provide a process-based characterization of river profiles. We propose that these geomorphic indexes provide a useful context for classifying basins that express different reach morphologies, fish habitat capacity, and responses to episodic disturbance. Strongly concave profiles that develop in steep terrain indicate that almost all of the relief in the drainage network occurs in small headwater streams. In these basins a large proportion of the drainage network has low-gradient morphologies, such as pool-riffle sequences, which provide favorable rearing habitat for many salmonid species. Complex metapopulation structures can develop within these networks because fish distribution expands into the tributaries, allowing for a spatial spreading of risk that may enhance a population's ability to persist during adverse conditions for survival and growth. The severity of pulse disturbances is also reduced because debris flows typically form discrete deposits where steep tributaries abruptly encounter low-gradient mainstem channels at tributary junctions. In contrast, less concave profiles in steep terrain indicate that the spatial extent of high gradient reaches morphologies, such as step-pool and cascade sequences, are more extensive. Metapopulation development in these basins is diminished because most tributaries are too steep to provide habitat, confining fish to mainstem channels. Furthermore, the change in slope at tributary junctions is less pronounced and debris flows rarely form discrete deposits. Instead, these mass flows continue to travel down steep mainstem channels and alter aquatic and riparian habitats for long distances. The combined influence of a limited spatial distribution and the increased severity of debris flows may result in more extreme fluctuations in population abundance because they are less resilient to pulse disturbances.
B42A-06 11:45h
Wildfire, channel disturbance, and stream temperature: spatio-temporal patterns and associations with the distribution of fish and amphibians in central Idaho
Temperature is a critical factor in stream ecosystems, and one that is very likely to be altered by wildfire and associated channel disturbance. In central Idaho streams, temperatures after wildfires may increase following loss of shade from riparian vegetation, and changes in channel structure that increase exposure to solar radiation and decreased hyporheic exchanges. To examine the spatio-temporal aspects temperature in relation to these influences, we employed three approaches: a long-term pre-post fire comparison of temperatures between a pair of streams, one burned and one unburned; a short-term pre-post fire comparison of a burned and unburned stream with spatially extensive data; a short-term comparative study of spatial variability in temperatures using a "space for time" substitutive design. These three approaches provided key insights into the value of each study approach and revealed some expected and some surprising associations between temperature and occurrence of native trout and tailed frogs. To further understand potential mechanisms influencing stream temperature, we used field-validated estimates of solar radiation to model the effects of riparian shade from remotely sensed vegetation data. These models confirmed the importance of riparian influences linked to wildfire for stream temperature in our study system. The collective results of this work highlight the importance of spatio-temporal variability in study designs to quantify the effects of wildfire and disturbance on stream temperatures, and the implications of stream temperature for aquatic species in a broad landscape context.