A: The main reason is the time needed for the rainfall water to reach the channels either by surface runoff or by underground flow.

In more words:

Let us consider that the rain is falling with the same intensity throughout a particular river basin. From all of the rain water a considerable portion of it evaporates while flowing over land or in channels. Some of the rain falls on the trees and bush and house roofs and evaporates, or with a time lag eventually reaches the ground. Some of the rain is falling directly onto rivers (channels) and is therefore named channel precipitation. Much more is overland flow, i.e. portion of the (rainfall) water flowing from wherever the droplets fall downhill to the channels. The time needed for a droplet to reach a channel depends from various factors such as the channel density, length of the paths (near or far from a channel), the slope inclination, slope ruggedness, vegetation, ground saturation etc. So there is a lag. Moreover the lag is not the same in every part of the river basin. Even the form of the river basin is important. If the river basin is narrow and elongated the rainfall water from all over the basin is reaching the main channel at roughly the same time and thus the water level will rise quickly, but along the whole length of the channel and not dramatically - low possibility of floods. The water level will also fall quickly after the rain stops. On the other hand if the river basin is large (developed) in its upper parts and is narrow in its lower parts - like having a big head on a long neck - then all the channels will bring its waters roughly at the same time in the central part of the "head" and there will be a dramatically water level rise in the main channel. This water wave will then travel along the lover portion or the "neck" of the basin with high possibility of floods.

So far we have discussed only surface runoff (from rainfall directly included into surface flow) but there is also subsurface runoff. Large portion of rain (not evaporated) infiltrates into underground. This is important because for your question because water flows very slowly under ground considerably widening the time lag. Some of the water is flowing subsurfacely, not reaching groundwater, but laterally down the slopes towards channels. This is called interflow which can reach up to 85% of direct runoff. This is one of the reasons for the time lag between the peak of rainfall and peak of flow in rivers. Some of the water infiltrates down to the groundwater. Groundwater is usually connected with the river beds and when the groundwater level is high (second half of rainy season and some time afterwards) it feeds the rivers, making them permanent. This is the answer to the question bothering Greeks in ancient times: Why do rivers flow when there is no rain to feed them - and their springs? In those days they thought that the Earth is a disc floating on the Ocean, and so they thought that the springs must be connected with the Ocean beneath the Earth disc and that the sea water rising upwards trough the disc looses its salt along the way. So, they thought, the rivers flow back to the Ocean but do not rise its level - the imagined circle was closed. Only later did their knowledge improve and they realized the concept of evaporation.

And some point for the end: We have not considered that there is precipitation other than rain, including snow. Snow can lay long on the ground and melt only in spring time. Also some rivers are feed by glaciers. And last but not least if there is karst in the river basin, or even the whole basin is in karst region majority of precipitation ends under ground, flowing trough a maize of underground channels ending up (or not) in various parts of river bed after a time lag. This is for instance typical for southern half of my country - Croatia.

I hope I gave You some good answers in bad English :-),

Dr. sc. Danijel Orešić