Large rivers are represented by a light blue area, while most smaller rivers are represented by lines.
Dashed lines correspond to non-perennial rivers. These rivers do not always flow (actually some are only flowing a few days per year, and in some areas, not even every year).
Water normally flows in perennial rivers all over the year, but in an exceptionally dry season, even a river indicated as perennial on the map might be dry.
In some plains, the water doesn't always flow at the same place, but there are many channels, sometimes scattered on a few hundredths meter. Such a situation is depicted like this on a map (the example does not correspond to the photo).
It is often important to determine the direction of flow. In most situation, this can be derived from the contour lines / spot heights (water naturally flows from higher to lower).
The top part of a river (or stream) usually corresponds to an easily recognisable organisation of contours, depicting the head of a valley:
The organisation of the network is also a good indirect indication of the direction of the flow: most streams converge towards the more important rivers (which are always lower), and the network has a tree-like organisation.
In most cases, the angle of the convergence of a secondary river with a bigger river can indicate the direction of flow in the main river, but this is not always the case.
When possible, the river names are placed in a position where the direction of the text follows the direction of the flow. If not possible, the direction of flow may be indicated by a small direction sign (this convention was developed for Namibian maps and it is not common to all topographic maps).