Over the years, accurate prediction of streamflow in both space and time has been a challenge. Identifying trade-off between the forecast time-horizon, basin scale, and streamflow forecasting accuracy is a work in progress. Due to interconnections of uncertainties in hydrologic processes and streamflow predictability, one needs to approach streamflow forecasting in a holistic framework. My dissertation investigates streamflow predictability across space and time scales using a series of data-driven and process-based methods of streamflow forecasting.