Water can obviously be collectd from existing streams or rivers, although some care may need to be taken if large populations or industries exist up-stream fromyou!!!
Some possible ways to collect this water are
This system taps the flow from an existing waterfall. In a dry part of Mexico, the systemis dry for much of the year, however there is sufficient water during the rainy season. Most of the water continues on its path, but some is diverted by the simple concrete wall into a large ferro-cement storage tank. Ref. huehue
In this diagram we are not actually in the stream bed, but obviously collecting the same source of water. This approach minimises problems where the level of the river changes rapidly due to floods etc.