What type of stream valley forms in mountainous areas?
Steep V shaped sided valleys unless in glacial areas; in this case the stream will be at the bottom of a large U shaped valley.
Usually a stream in mountainous areas follows some lineament of weakness in the rocks, such as faults, eroding the landscape to form its bed.
The energy of the water is higher when a river flows through the mountains because of the steeper altitude gradient. As a result the river will flow faster eroding deep "V" shaped valleys or gorges.
In some areas (as example the Highlands in Scotland) the rivers, even in mountainous areas, flow on the bottom of large "U" shaped valleys. This is because these valleys were carved by glaciers during the last ice age and now water flows within them.
Once reached the plain area at the foothill of the mountains the river will change its morphology becoming slower and likely forming a braided pattern with several shallow channels that divide and recombine.