I can't remember where I found this, but it's a handy guide for how river systems tend to form (this is based on Californian systems.)

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Note the footnotes - Parallel, Annular, and Rectangular are uncommon—at least in California. Distributary systems form when a river reaches sea level and breaks apart. Because of modern engineering and consisting dredging to keep shipping lanes open, this fracture doesn't happen as often as it used too.