Definately!   To give your leash a freshening up after a dousing in the river, a drag through the mud, and all the other fun places your dog's adventures take him, try a good rinse first- the hose if fine.  If the dirt is stubborn, use a bristle (not metal) brush and dish or laundry soap.  Start scrubbing gently with cold water.  If the dirt isn't leaving, scrub harder with warmer water.  It is inevitable that the leashes don't look new forever, but most of the dirt should come off.  Rinse in cold water and hang it up to dry.