Most likely the abs sensor's or the harness plugs to the sensors have got water in them and that will turn on the abs light as the water can short the pins and give the abs controller an open or shorted code, while you will still have normal braking you wont have anti lock braking, you can unplug each abs sensor and check the sensor individually I think the reading is 18k-25k ohm's but it would be best to have someone hook up a scanner to see which wheel it is. Here in Ohio most places like autozone and advanced and Napa's can scan them for free.
__________________
|