Fluctuations in water surface elevations (WSEs) along rivers have important implications for water resources, flood hazards, and biogeochemical cycling. However, current hydrodynamic modeling and remote sensing methods exhibit key limitations in characterizing spatiotemporal hydraulics in many of the world’s river systems, particularly multichannel rivers. This dissertation investigates the capabilities of state of the art hydrodynamic models and new remote sensing observations to characterize surface water dynamics across a large-scale, anabranching river. In Chapter 1, I build and compare six different hydrodynamic models along the Tanana River, AK to investigate (1) how well a simple, raster-based model can simulate 2D channel hydraulics, and (2) how degrading the physical representation of a multichannel river system affects spatial and temporal errors in model outputs. I show that simple, raster-based models can accurately simulate 2D, in-channel hydraulics in a complex multichannel river, and that inclusion of the anabranching network is essential for simulating proper hydraulics at regional-scales. In Chapter 2, I validate new radar measurements of WSE and slope from AirSWOT, an airborne analogue to the Surface Water and Ocean Topography (SWOT) mission. I find that AirSWOT accuracies are at least as good as what we expect from SWOT, and in some cases, substantially better, with RMSEs of 9.0 cm for river WSEs when averaged over 1 km2 areas, and 1.0 cm/km for slopes along 10 km reaches. In Chapter 3, I investigate AirSWOT’s ability to capture multi-temporal fluctuations in WSE and slope. I demonstrate that AirSWOT can provide a comprehensive picture of river dynamics by observing decimeter-level WSE changes when averaged over 1 km2 areas and centimeter-per-kilometer-level slope changes for reaches ≥5 km with RMSEs of 10.4 cm, and 1.0 cm/km respectively. Additionally, AirSWOT measurements add marginal differences when estimating discharge with an RMSE of 15.3% compared to in situ discharge estimates and 42% of AirSWOT discharge estimates falling within the in situ discharge uncertainty. In the future, AirSWOT measurements can be used to study detailed passages of flood waves, understand how features of riverbeds and banks affect patterns of flow, and integrate into local and regional-scale hydrodynamic models to improve flood predictions.