Abstract
This field study investigated how oyster canopy density modulates hydrodynamics of natural intertidal oyster reef. High‐resolution velocity measurements were taken simultaneously within and above oyster canopies of variable density (solid volume fractions, 0.04–0.21). Flows responded monotonically to increasing canopy density up to a threshold ( ≈ 0.2), beyond which nonlinear transitions in flow–reef interaction were observed. For instance, flow attenuation and drag coefficients () above the canopy increased with density until (mean attenuation 48–65%, 0.02–0.08). Beyond this threshold density, mean flows accelerated by 60%, and decreased to ~ 0.01. Similarly, turbulent production and dissipation () increased with canopy density, by factors of 2–3 above moderately dense and 7–19 above the densest canopies relative to sparse and bare‐bed conditions. Above the densest canopy, production and dissipation were, respectively, 25 and 53 times greater than turbulence observed within the canopy. Mean and turbulent flow patterns pointed to formation of a high velocity shear layer above the densest canopy, isolating the canopy density at which flows were preferentially routed above the canopy. Water level modulated these relationships: all canopies suppressed flows uniformly during seasonally low water levels, while distinct density‐dependent flow patterns emerged under higher water levels. Sediment analysis revealed accumulation of finer particles (0.3–12.4% silt/clay) and elevated organic content (up to 86.2 g kg −1 ) in dense canopies. Overall, canopy density emerged as a driver of small‐scale hydrodynamics with important implications for mass transport in restored and natural reefs.