Rivers are part of the hydrological cycle; they ‘complete’ the hydrological cycle studied before by concentrating the flow of water precipitated on the surface and taking it to the ocean or endorheic lakes (Bradshaw and Weaver 1995). Thus, rivers like the Gina river in Ethiopia develop where precipitation exceeds evapotranspiration plus infiltration. This applies also for only periodically or episodically surpluses. When this occurs depends on the characteristics of a watershed (e.g., bedrock).
The Gina river receives the water which fell as precipitation and circulated through the drainage basin. Additionally, the river contains the soil and rock fragments which were eroded from hill slopes. Loaded with these materials the Gina river erodes its river bed and banks. The river transports the material emanated from weathering and erosion on slopes and load of its own activity. Along the river course tributary streams join the river at confluences. These add water along the length of the river. Where the ability of the river to transport material decreases the transported load is deposited as fluvial sediment (Bradshaw and Weaver 1995; Marcinek 1997).
To understand river processes it is essential to know about the discharge behaviour which is explained on the following page.