ABSTRACT
Flows are an essential part of the hydrological cycle. The water precipitated onto a watershed enters one of four processes: it is intercepted, it evaporates, it infiltrates the soil, or it enters the process of flow. This chapter is devoted to the last element, and the connected process of infiltration. The quantity of water that collects and travels in a river comes from either direct precipitation onto the waterway or from surface, subsurface and groundwater flow. We also distinguish between the water that infiltrates and then finds its way slowly into the groundwater and aquifers towards the outlet as groundwater flow, and the water that moves quickly towards the outlet to constitute the flood, sometime called rapid flow, which involves the water moving on the surface or just below the surface (subsurface).
