The "Y" shape of the water is caused by the water "splitting" around the obstacle. Another word would be "bifurcate" which means splitting into two paths (such as bifurcation of the water flow), but that may be too technical sounding. If that is the case, the path of the water is certainly "parting". Or, less directly, one could simply state that the stream "surrounded" him (diverging around him naturally - thus splitting is implied).
"He stood in the middle of the shallow river, the water splitting around him."
"He stood in the middle of the shallow river, the water bifurcating around him."
"He stood in the middle of the shallow river, the water parting about him."