You mean gullies dont think its that easy. Wide river entering zero changes stream to swamp but dry streams only way I know set base terrain to level 1 or higher & make a 1 hex wide depression probably filling with rough. If set hill base size large with height of 1 it will make some for you if play a bit.
Long way as have to do each hex individually but use the asterix button. Suggest place any terrain features in it & adj hexs first then adjust height. Dont try & get to clever or you will end up with invisible slopes.
Plus that the -1,-2 or -3 "Levels" are not height levels but probably a "code attribute"(?) to diversify stream, shallow and deep water hexes from level 0 ground terrain?