Water Velocity and Discharge from Tidal Freshwater Creeks in Forested and Herbaceous Wetlands

Acoustic Doppler Current Profilers were deployed in four tidal creeks within forested swamps and herbaceous marshes in the lower Columbia River. Measurements were collected over 12 to 20-day periods during high and low river stage, and then analyzed using wavelet tidal analysis software (CWT_Multi) to characterize temporal variability of water elevation, velocity, and discharge. While subject to similar fluvial and tidal forcing, nuances in creek geometry led to large differences in their hydraulic response. Most obviously, creeks in the higher elevation forested swamps were hydraulically isolated, which limited subtidal discharge but increased tidal monthly variability of discharge in comparison to the lower elevation marsh sites. Discharge in the marshes was ebb-dominant (3:1 ebb: flood ratio), while discharge at the forested sites was slightly flood dominant during neap tides and ebb dominant during spring tides. Net discharge over a 12-day period exceeded 1,000,000 m3 for the marsh sites but was less than 200,000 m3 for the forested sites. These transport patterns set limits on sediment dynamics, fish ingress, and material fluxes from these habitat types. Comparison to the literature on tidal creeks highlights the distinctive nature of tidal freshwater wetlands in mixed-tide environments of the lower Columbia River, which are more ebb-dominant (due to diurnal-semidiurnal tidal phase relationships), fluvially influenced, and temporally variable (due both to fluvial input and non-stationary tidal asymmetry) than saltmarsh counterparts in semidiurnal environments. Finally, we demonstrate how wavelet analysis of water levels can be used to extend velocity and discharge records, thereby facilitating long-term monitoring of wetlands.