I have a contour map. I want to create a drainage map from it and try to read the each pixel value of the drainage map and rectify the map using a threshold value. how can i do this?

i hve a contur map. now i want to create a drainage map using the contour map and read the pixel value of the drainage map. after that want to rectify the map using a threshold value under which the small drainage network will be removed and create a main stream drainage map.