Mark S. Lorang

The shifting habitat mosaic of river ecosystems

The essence of ecology is to understand the distribution and abundance ofbiota (ANDREWARTHA & BIRCH 1954). In the same vein, a comerstone of ecology is quantifying how and why organisms are dependent on specific biophysical space (habitat) to complete one stage or another in their life cycles (SOUTHWOOD 1977). On the one hand, phenotypic plasticity promotes successful growth and reproduction in variable habitats, but on the other hand habitat fidelity over several to many generations may constrain (adapt) the species or life stage to a habitat with quite specific spatial or functional attributes. Conservation biologists sometimes refer to these locally adapted populations with habitat-specific life cycles as ecologically significant units. Such populations have been accorded special protection and management ifthey are rare or declining in numbers. However, habitat intrinsically is not static, owing to constantly changing successional (or gradient) states as landscape is mediated by interactive physical ( e.g. flood, drought, fire) and biological (e.g. disease, predation, invasion) drivers. Thus, physical and biological attributes vary in time and space and interact to determine quantity and quality of specific habitat per life stage. Sufficient quality habitat is required to permit a positive life history energy balance to sustain a population over the long term, otherwise extinction occurs (HALL et al. 1992). Particular species, and even particular populations of species, either adapt to the dynamic nature o f habitat or they fai! to persist in that landscape. Of course, a given landscape is composed of n-dimensional gradients and species responses, and feedbacks are complex and nonlinear, making habitat per life stage o f each species in the landscape very difficult to define. Nonetheless, quantifying habitat for species in very specific spatial and temporai terms is fundamental to conservation o f biodiversity world wide.

CLIMATE, HYDROLOGIC DISTURBANCE, AND SUCCESSION: DRIVERS OF FLOODPLAIN PATTERN

Floodplains are among the worlds most threatened ecosystems due to the pervasiveness of dams, levee systems, and other modifications to rivers. Few unaltered floodplains remain where we may examine their dynamics over decadal time scales. Our study provides a detailed examination of landscape change over a 60-year period (1945-2004) on the Nyack floodplain of the Middle Fork of the Flathead River, a free-flowing, gravel-bed river in northwest Montana, USA. We used historical aerial photographs and airborne and satellite imagery to delineate habitats (i.e., mature forest, regenerative forest, water, cobble) within the floodplain. We related changes in the distribution and size of these habitats to hydrologic disturbance and regional climate. Results show a relationship between changes in floodplain habitats and annual flood magnitude, as well as between hydrology and the cooling and warming phases of the Pacific Decadal Oscillation (PDO). Large magnitude floods and greater frequency of moderate floods were associated with the cooling phases of the PDO, resulting in a floodplain environment dominated by extensive restructuring and regeneration of floodplain habitats. Conversely, warming phases of the PDO corresponded with decreases in magnitude, duration, and frequency of critical flows, creating a floodplain environment dominated by late successional vegetation and low levels of physical restructuring. Over the 60-year time series, habitat change was widespread throughout the floodplain, though the relative abundances of the habitats did not change greatly. We conclude that the long- and short-term interactions of climate, floods, and plant succession produce a shifting habitat mosaic that is a fundamental attribute of natural floodplain ecosystems.

Flow competence and streambed stability: an evaluation of technique and application

The ability to assess flow hydraulics and the movement of substrata is important to understanding the complexity of habitats and variation in spatial and temporal conditions in stream and river ecosystems. The concept of flow competence (i.e., flow necessary to mobilize the streambed) has been applied to analyses of streambed stability and generally is based on the assumption that measures of rock size composing the substrata can be used to estimate the magnitude of flow that would mobilize the bed material. However, analyses have had limited success owing in part to the expectation that simple linear expressions accurately predict threshold entrainment (i.e., initiation of bed movement) over a broad range of hydraulic conditions from low-gradient sand-bed rivers through high-gradient boulder-bed rivers. Examination of the literature revealed ambiguity of terminology and use of equations without the proper adherence to limitations and assumptions inherent in the derivation of the equations. We present the derivations of the founding equations of flow competence, and discuss their applications, assumptions, and limitations. We also examine the efficacy of a stream stability index based on the ratio of estimated bottom boundary shear stress applied to the bed during bankfull conditions, and the critical entrainment shear stress calculated from the size of the surface bed material. Data from 33 gravel- to boulder-bed river sites were used to evaluate the stream stability index as a predictor of bed movement. The stability index indicated that a bankfull discharge would not generate critical threshold entrainment for 28 of the 33 sites, with most sites requiring a 2-fold increase in bottom boundary shear stress to mobilize the riverbed gravel and cobbles, and nearly an order of magnitude increase in shear stress to mobilize boulders. We conclude that other factors affecting the possible range of variation in flow velocity, particle size distributions, bed packing, and momentum exchange from collisions between saltating particles and those at rest on the bed may lead to departures between predicted and actual streambed movement. Hence, flow-competence approaches to predicting streambed instability should be used only to objectively bracket ranges of variation in disturbance and streambed movement. Moreover, the assumptions and limitations of equations for such indices should be investigated carefully before elaborate statistical analysis of watershed or streambed variables are undertaken to explain deviations from theoretical estimates, or to explain why a particular index does or does not predict bed movement.

USING AIRBORNE MULTISPECTRAL IMAGERY TO EVALUATE GEOMORPHIC WORK ACROSS FLOODPLAINS OF GRAVEL‐BED RIVERS

Fluvial processes of cut and fill alluviation and channel abandonment or avulsion are essential for maintaining the ecological health of floodplain ecosystems char- acteristic of gravel-bed rivers. These dynamic processes shape the floodplain landscape, resulting in a shifting mosaic of habitats, both above and below ground. We present a new and innovative methodology to quantitatively assess the geomorphic work potential nec- essary to maintain a shifting habitat mosaic for gravel-bed river floodplains. This approach can be used to delineate critical habitats for preservation through land acquisition and conservation easements, often critical elements of river restoration plans worldwide. Spa- tially explicit modeling of water depth, flow velocity, shear stress, and stream power derived from surface hydraulic measurements was combined with airborne multispectral remote sensing for detailed modeling of floodplain water surfaces over tens to hundreds of square kilometers. The model results were then combined within a GIS framework to determine potential nodes of channel avulsion that delineate spatially explicit zones across the flood- plain where the potential for geomorphic work is the greatest. Results of this study dem- onstrate the utility of integrating existing multispectral remote sensing data coupled with time-lagged ground-based measures of flow hydraulics to model fluvial processes at rela- tively fine spatial resolutions but over broad regional extents.

Predicting the crest height of a gravel beach

The beach crest is a common morphological feature formed by the deposition of sediment carried up-slope by wave swash. The elevation to which waves can pile gravel is a function of the size and density of the material relative to the hydraulic components of swash velocity, wave frequency and runup height. Two equations ((17) and (21)) that predict the height of the beach crest to the wave forces and the beach material are derived. The first derivation compares the wave force acting to move a stone up the beach face with a weight force acting to hold the stone in place. The second derivation relates the potential energy per unit area of the beach crest to the total wave energy that lifted and deposited the material above a given sea-level datum. As a test of the derived equations, the actual crest height of one natural gravel beach was accurately estimated by both equations derived. Further research and testing on other natural beaches are needed to fully verify Eqs. (17) and (21). A comparison of other equations (Eqs. (2), (3) and (4)) that predict crest height is also made. The poor fit of the other equations are a result of using values for the coefficients that were established in wave tank experiments. Another shortcoming of Eqs. (3) and (4) is that they do not include variables related to the beach material. Hence, they predict that all particle sizes from sand to boulders would be piled to the same elevation by the same wave forcing. Eq. (2) includes a particle size variable but predicts increasingly higher beach crests for larger particles. More realistic results for all three equations (Eqs. (2), (3) and (4)) would be expected with field-tested values for the coefficients, but a range in values for each different beach would be necessary. The advantage of Eqs. (17) and (21) is that equations are not dependent on coefficients whose values come from correlation with other data sets.

Fluvial Geomorphic Processes

Abstract Transport of sediment by moving water is a fundamental feature of earth processes. One of the primary interdisciplinary aspects of stream ecosystem structure and function is the relationship between flow dynamics and the movement of the substratum of stream and river channels. Stream flow variance plays a central role in the structure and function of stream ecosystems. Near-bed flow velocity, streambed instability, and scour induce a subsequent ecosystem response that often exerts density-independent effects on stream, as well as riparian, organisms. These changes in stream discharge, flow velocity, and shear stress cause spatial variability in fluid forces and thus differential entrainment, transport, and deposition of sediment. There is a need in stream ecology for a condensed derivation of basic equations used to describe shear stress and incipient motion, coupled with an analysis of streambed stability and the utility of deriving stability indices. This chapter introduces the concepts of flow-competence, shear stress, and the fundamental processes in threshold entrainment of bed material in river systems.

Dissipative and reflective beaches in a large lake and the physical effects of lake level regulation

Abstract Nearshore environments in Flathead Lake, Montana, USA, were described as dissipative or reflective on the basis of: the surf similarity parameter ϵ, grain size, morphology, number of breaking waves and angle of wave incidence. The relative resistance to foreshore and backshore erosion caused by anthropogenic lake level regulation was compared between these two nearshore configurations. Reflective systems were characterized by dynamic gravel beach faces and steep inshore shelves armored by wave-washed cobble. In contrast, dissipative systems were characterized by sand-sized substratum, broad flat inshore shelves and the presence of multiple linear bars approximately 350 m offshore. Five decades of regulated lake levels have resulted in extensive shoreline erosion (970 ha on the north shore of the lake) and a general reshaping of both types of nearshore environments, although dissipative shorelines eroded faster (5.7 m/year maximum and 2.0 m/year average). The presence of docks and other man-made structures on reflective beaches accelerated erosion by intercepting longshore gravel transport. This analysis provided a physical basis for understanding the effects of lake level regulation on shoreline ecology and management.

Combining active and passive hydroacoustic techniques during flood events for rapid spatial mapping of bedload transport patterns in gravel-bed rivers

With 7 figures Abstract: Turbulent flow in rivers and the associated movement of sediment creates unique underwater sound- scapes that can be measured passively with hydrophones while Acoustic Doppler Profilers (ADP) are an active form of hydroacoustic sampling that can be used to provide a surrogate measurement of Apparent Bedload Veloc- ity (ABV). In our study, longitudinal profiles of ADP and sound were simultaneously measured, while floating the river in a raft on the Nyack Floodplain of the Middle Fork of the Flathead River, USA during flood events exceeding bankfull conditions. In addition, similar measurements were carried out on the Kootenai River during a prescribed flood release aimed at mobilizing gravel bed sediment to positively impact White Sturgeon spawn- ing. Both data sets revealed spatially explicit zones of coherent ABV and bedload intensity (sound) over the two 12 km river segments. The ability to remotely and in real-time assess bedload transport for large gravel-bed rivers, on the floodplain scale, is a missing piece of information important for basic ecological understanding and applied science, specifically management decisions regarding regulated rivers worldwide. With these data sets we dem- onstrate a new methodology for rapid real-time spatial surveying of bedload transport in large gravel-bed rivers.