Miscellaneous
Orography: The descriptive terms right or left side-valley (resp. tributary) always apply from the perspective of looking downstream (in the direction the current is going).
Sandbanks: often at reaches with slow current, especially near the river banks. Looking at the various rock types of the gravel is an excellent and cheap survey for a summarized geology of the rivers watershed (catchment area).
Valley forks: at most confluences it is clear which stream is the main river and which is the tributary - as one stream is both much longer and carrying clearly more water than the other. It can, however, happen that one stream is longer, but the other carries more water. This case has no fixed rules to decide. Usually the longer valley is chosen as the main valley, e.g. for the higher number of villages.
But in Lowlands the decision may be arbitrary: In Switzerland the long Alpine Rhine is chosen as the main stream, although the Aar carries more water for her huge midland area.
In the case of the Mississippi River (which carries more water) and Missouri River (which is much longer) the choice is made according to the opposite criterion.
Read more about this topic: Side Valley