I have contours layer and I want to build a model to derive valleys from this layer
As you see in the screenshot bellow the red zone is a valley.
How I can derive all valleys. any help regard this?
you should begin to examine the hydrology tools and see about building a stream network... whether there is actually a stream there or not isn't relevant, but it will be useful in identifying areas where flow accumulation and direction could give rise to stream flow. The start of the menu tree and its underlying concepts begins here... http://desktop.arcgis.com/en/arcmap/latest/tools/spatial-analyst-toolbox/identifying-stream-networks...