confluence or conflux: where a two rivers join or a tributary joins a river (interesting examples here) Below, a normally mellow tributary (Slick Rock Crk.) races into the Salmon River (Oregon Coast). 

slick rock creek-Oregon-SwittersB
Slick Rock Creek in the foreground and the Salmon River beyond the point of land.

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conflux-Slick Rk Crk-Salmon R-Oregon-SwittersB-2
Here, the stream (creek) joins the river in the distance…a conflux