alter, change, modify - cause to change; make different; cause a transformation; "The advent of the automobile may have altered the growth pattern of the city"; "The discussion has changed my thinking about the issue"

foul - make unclean; "foul the water"

contaminate, pollute, foul - make impure; "The industrial wastes polluted the lake"

smear - stain by smearing or daubing with a dirty substance

slime - cover or stain with slime; "The snake slimed his victim"

muddy, muddy up - dirty with mud

splash - soil or stain with a splashed liquid

mud, muck up, muck, mire - soil with mud, muck, or mire; "The child mucked up his shirt while playing ball in the garden"

crock - soil with or as with crock

blemish, spot - mar or impair with a flaw; "her face was blemished"