Physical Structure
The physical structure of a river corridor encompasses the three-dimensional arrangement of its geomorphic features, hydraulic conditions, and substrate materials. This structure forms the physical template ecological communities develop and interact. Key components include the topography of the floodplain, the hydraulic characteristics within the channel, the arrangement of channel bedforms, and the composition of the streambed substrate. These elements determine the availability, diversity, and quality of physical habitats for aquatic and riparian organisms.
Component: Floodplain Topography
Characteristics: The configuration of the land surface adjacent to the river channel that is periodically inundated by floods. Includes features such as:
Natural Levees: Slightly elevated ridges immediately adjacent to the channel, formed by deposition of coarser sediment during overbank flows.
Backswamps/Flats: Lower-lying areas further from the channel, often accumulating finer sediments.
Terraces: Abandoned, older floodplain surfaces sitting at higher elevations, rarely inundated under the current flow regime.
Relict Channels: Abandoned meander loops or channels forming depressions or standing water bodies on the floodplain surface.
Scroll Bars: Gently undulating ridges and swales formed by past point bar deposition during lateral channel migration. The overall topographic complexity or heterogeneity is a key attribute.
Ecological Influence: Floodplain topography dictates the pattern, depth, and duration of inundation during floods, creating a mosaic of hydrologic conditions. This hydro-topographic complexity supports diverse plant communities adapted to different flood frequencies and soil moisture regimes. It creates a variety of habitats for fish, amphibians, birds, and mammals. Topographic lows retain floodwaters longer, supporting wetland habitats and influencing nutrient processing and carbon storage. Heterogeneity enhances overall biodiversity and ecosystem resilience.
Human Impacts:
Levees/Floodwalls: Artificially create high, uniform berms that disconnect the floodplain from the river and eliminate natural topographic variation related to flooding.
Land Leveling/Filling (Agriculture, Urbanization): Homogenizes floodplain topography, removing natural depressions, channels, and levees, reducing habitat diversity and altering inundation patterns.
Channel Incision: Lowers the riverbed relative to the floodplain, reducing the frequency and extent of inundation of existing floodplain features.
Dredging/Soil Deposition: Can alter floodplain elevations and create artificial topographic features.
Component: Channel Hydraulics
Characteristics: The properties of flowing water within the channel, primarily flow velocity, water depth, and boundary shear stress. These vary spatially across the channel cross-section and longitudinally, and temporally with changes in discharge. Hydraulic geometry describes the relationships between discharge, channel dimensions and velocity. Turbulence and flow structures are also important hydraulic characteristics.
Ecological Influence: Hydraulic conditions are primary determinants of habitat suitability for aquatic organisms. Different species and life stages have specific requirements or preferences for velocity, depth, and shear stress. For example, some macroinvertebrates require high velocities, while others prefer slow-moving pools. Fish select habitats based on velocity, depth, and shear stress. Hydraulic forces also govern sediment transport, bedform development, and scour/deposition patterns. Hydraulic diversity is key to supporting high biodiversity.
Human Impacts:
Flow Alteration: Directly changes discharge, which in turn alters velocity, depth, and shear stress patterns throughout the channel. Reduced flows decrease velocities and depths, while regulated or hydropeaking flows create unnatural hydraulic fluctuations.
Channelization/Straightening: Increases channel slope and confines flow, leading to higher average velocities and shear stresses, often simplifying hydraulic diversity.
Instream Structures: Create localized hydraulic changes, such as increased velocity through constrictions, flow separation, and altered turbulence patterns.
Removal of Roughness Elements: Reduces hydraulic complexity, leading to more uniform flow conditions.
Dredging: Alters channel depth and cross-sectional shape, directly modifying hydraulic geometry.
Component: Channel Bedforms
Characteristics: Morphological features formed on the bed of alluvial channels by the interaction of flowing water and mobile sediment. Common bedforms include:
Ripples: Small-scale features formed in sand and fine gravel at lower flow velocities.
