Quantifying geomorphic form variation for assessing habitat complexity of river restoration designs

Rivers have historically been simplified to facilitate navigation, transportation, and water flow management. A shift towards naturalizing river channels and incorporating principles of fluvial geomorphology and ecology have progressed with variations in restoration approaches but channel design remains largely driven by conventional stabilization methods, particularly in urbanized watersheds. These approaches simplify channel morphology to accommodate changes in flow and sediment regimes induced by catchment-scale urbanization and have led to calls for more process-based solutions to enhance the natural dynamics of geomorphic riverscape design and improve ecological integrity. To understand the effects of restoration design on channel morphological variety, we take advantage of high-resolution digital elevation models to evaluate 28 real-world river restoration design projects of various types by applying the Geomorphic Form Variation (GFV) approach to quantify surface-form complexity differences. Pre-existing and designed river channels were evaluated and changes in surface-form variety associated with restoration design methods and project types were measured. Project cases were categorized as 1) stabilization, 2) full channel realignment, and 3) habitat enhancement types and GFV values were illustrated with hotspot cluster maps for comparative assessment. Stabilization project cases showed decreased GFV values resulting from simplified morphological features, planform channel shape, and floodplain surface topographies. Full channel realignment cases largely showed greater complexity resulting from increased sinuosity and added floodplain topographic variation while habitat enhancement cases conclusively increased GFV values with irregular in-channel bed forms and floodplain connectivity components such as multi-thread channel patterns. We demonstrate how GFV can serve as a valuable tool in assessing restoration design and construction outcomes and guiding the planning process towards enhancing geomorphic variety and habitat heterogeneity. This approach can point to nature-based channel design solutions that improve geomorphic complexity as a measure of natural morphology conditions where conventional stability methods may fall short in addressing geomorphic and ecological concerns. • A new metric for evaluating channel design methods and geomorphic complexity • River design assessed by Geomorphic Form Variation (GFV) of 28 projects • Reduced (GFV) complexity common in stabilization design of channel simplification • Increased (GFV) complexity in habitat enhancement projects of irregular riverscapes • GFV can provide useful components of channel design assessment and planning