Corridor Dynamics
River corridors are naturally dynamic landscapes shaped by the interplay of flowing water, sediment transport, and vegetation over time. This dynamism is expressed through various geomorphic processes that continually modify the channel and floodplain environment. These processes operate across different timescales, from rapid changes during individual flood events to gradual evolution over decades or centuries. Key dynamic processes include lateral channel migration, channel bar formation and migration, riffle-crest migration, and channel avulsion. These processes are fundamental to creating and maintaining the physical habitat complexity and heterogeneity that supports biodiversity in healthy river ecosystems.
Component: Lateral Channel Migration
The gradual shifting of an alluvial river channel across its floodplain over time. It primarily occurs in meandering and wandering river patterns and is driven by erosion on cut and outer banks and concurrent deposition on point bars and inner banks. Flow patterns in bends direct higher velocity flow towards the outer bank, causing undercutting and erosion, while transporting eroded sediment across the channel to deposit on the slower-flowing inner bend. Migration rates vary depending on flow energy, sediment load, bank material erodibility, and riparian vegetation influence.
Lateral migration is a primary mechanism for floodplain turnover and renewal. Bank erosion supplies sediment and large wood to the channel, essential components for habitat structure. Point bar deposition creates bare, moist surfaces ideal for the establishment of pioneer riparian vegetation like cottonwoods and willows. The process maintains a mosaic of different-aged floodplain surfaces and vegetation communities, increasing overall habitat diversity. Abandoned meander bends can form oxbow lakes, providing important off-channel aquatic habitat. This dynamism prevents channel stagnation and promotes ecosystem resilience.
Human Impacts:
Bank Stabilization with Riprap and Concrete: Directly prevents bank erosion and halts lateral migration. This eliminates sediment and wood input from bank sources, prevents floodplain renewal, reduces habitat dynamism, and can transfer erosional energy downstream, potentially causing instability elsewhere. Widespread in developed areas and along infrastructure corridors.
Channelization/Straightening: Eliminates meander bends, thereby stopping lateral migration processes associated with bend dynamics.
Flow Regulation: Decreases the frequency and magnitude of flows capable of causing significant bank erosion and channel movement, slowing or halting natural migration rates.
Levees: Confine the river, preventing it from accessing its floodplain and migrating freely across the valley bottom.
Riparian Vegetation Removal: Increases bank erodibility and potentially accelerates migration rates initially, but loss of mature trees reduces long-term stability and wood recruitment.
Component: Channel Bar Formation and Migration
Bars are accumulations of sediment (sand, gravel, cobbles) deposited within or along the margins of the river channel where local flow velocity and transport capacity decrease. Point bars form on the inside of meander bends due to lower velocity and deposition of sediment eroded from the outer bank. Mid-channel bars form within the channel, often initiated by deposition around obstructions or where flow diverges and loses transport power. They cause flow to split around them and are characteristic features of braided rivers. Bars create topographic and hydraulic diversity within the channel. Exposed bar surfaces provide habitat for terrestrial invertebrates and nesting sites for some birds. Bar margins and submerged portions provide shallow water habitat for fish and invertebrates. Bars influence flow patterns, contributing to the formation of adjacent pools and riffles. They are key sites for riparian vegetation colonization. The constant shifting of bars in dynamic systems like braided rivers maintains early successional habitats.
Human Impacts:
Flow Regulation: Decreases the flows needed to mobilize sediment and rework bars, leading to stabilization and vegetation encroachment on formerly active bar surfaces. Can reduce the overall area and dynamism of bar habitats.
Sediment Supply Alteration: Reduced sediment supply below dams can lead to the erosion and loss of downstream bars. Excess sediment supply from land use can lead to bar growth and channel aggradation.
Channelization/Bank Stabilization: Eliminates the meander dynamics that form point bars and can simplify channel form, reducing bar formation overall.
Gravel Mining: Direct removal of bar features destroys habitat and alters local sediment budgets and channel form.
Component: Riffle-Crest Migration
Riffles are shallow, high-gradient sections of a channel characterized by coarser substrate and turbulent flow, typically alternating with deeper, lower-gradient pools in pool-riffle sequences. These sequences are common bedforms in gravel-bed rivers with moderate gradients. The riffle crest is the topographic high point of the riffle, often acting as a local hydraulic control, particularly at low flows. Riffles are depositional features formed during high flows when pools are scoured. As channels migrate laterally or sediment pulses move through the system, riffles and their crests can gradually migrate downstream. Sediment transport patterns are complex: at high flows, pools scour and riffles may aggrade or transport sediment through, while at lower flows, riffles can be sources of sediment that deposit in pools. Some evidence suggests sediment may bypass pools by moving over adjacent point bars during high flows.
Pool-riffle sequences create significant habitat heterogeneity, providing distinct environments for different species and life stages. Riffles offer well-oxygenated, shallow, fast-flowing habitat with coarse substrate, ideal for many macroinvertebrates and fish spawning. Pools provide deeper, slower water refuge during low flows or temperature extremes. The undulations drive hyporheic exchange, influencing water quality and temperature. The dynamic migration of these features contributes to the overall shifting habitat mosaic.
Human Impacts:
Flow Alteration: Changes in the magnitude and frequency of high flows disrupt the natural scour and fill processes that maintain pool-riffle morphology. Reduced peak flows may prevent pool scouring, while altered baseflows can affect low-flow hydraulics.
Sediment Regime Alteration: Excess fine sediment can fill pools and embed riffle substrates, degrading habitat quality. Sediment starvation below dams can lead to coarsening or armoring of riffles and potential incision, altering the pool-riffle structure.
Channelization/Dredging: Directly removes or simplifies pool-riffle sequences, homogenizing the channel bed.
Instream Structures: Can artificially create pool-like features but disrupt natural riffle formation and migration processes.
Component: Channel Avulsion
A relatively abrupt and often large-scale shift in the course of a river channel, where the river abandons its existing channel to establish a new path across its floodplain. Avulsions are a fundamental process in building alluvial landscapes like floodplains, alluvial fans, and deltas. They typically occur when the existing channel aggrades to the point where it sits higher than the adjacent floodplain, creating a slope advantage for flow to find a new, lower path, often during a large flood event that breaches a levee or bank. The process can involve incision into the floodplain or progradation of a new channel from a crevasse splay. Avulsion frequency is linked to sedimentation rates and channel morphology. While major avulsions are infrequent, smaller-scale channel shifts and cutoff avulsions are more common.
Avulsions are major disturbance events that dramatically reorganize the river corridor landscape. They create new channels, abandon old ones, redistribute sediment across large areas of the floodplain, and initiate new cycles of riparian vegetation succession. This large-scale resetting process is crucial for maintaining long-term floodplain heterogeneity and ecosystem complexity, particularly in depositional environments like deltas and alluvial fans.
Human Impacts:
Levees and Embankments: Constructed specifically to prevent avulsions and confine rivers to their current course and halt critical landscape-building processes.
Dams: By reducing downstream sediment loads, dams decrease aggradation rates in the main channel, which can reduce the natural trigger for avulsions in depositional zones.
Channel Dredging/Maintenance: Artificial lowering of the channel bed can counteract natural aggradation and reduce avulsion potential.
Land Use Change: Increased sediment supply from watershed disturbance can accelerate channel aggradation, potentially increasing avulsion frequency or magnitude if the river is not confined.
Management Implications
The inherent dynamism of river corridors, driven by processes like migration and avulsion, is often viewed negatively from a human perspective seeking landscape stability for infrastructure, agriculture, or property boundaries. Consequently, common river management practices like bank stabilization and channelization aim to suppress these natural dynamics. This suppression leads to ecological degradation by reducing habitat heterogeneity, simplifying channel form, disconnecting floodplains, and diminishing ecosystem resilience. Recognizing that dynamism is essential for health, process-based restoration approaches increasingly advocate for restoring or accommodating natural channel dynamics, such as allowing for lateral migration within defined corridors or reconnecting floodplains, rather than imposing static channel designs. Letting the river erode and migrate, within acceptable bounds, can lead to more diverse, self-sustaining, and resilient ecosystems. This requires a shift in management philosophy from controlling the river to working with its natural tendencies.
