We want to know how combined snow water equivalent (SWE) and streamflow data assimilation (DA) impacts streamflow predictions and how it compares with DA of SWE only and streamflow only. Results show that streamflow DA positively impacts short-term streamflow forecasts, while the impact of SWE DA lasts over a much longer horizon. Simultaneous DA of both SWE and streamflow performs better than its individual counterparts at all forecast horizons.