Always check this site for predicted weather conditions and flash flood warnings/advisories before traveling in the canyons. More detailed information can be obtained just prior to your trip at the Paria Ranger Station.
Flash Floods
Flash floods can occur during heavy rainstorms any time of the year, but especially during July, August and early September. These unpredictable events can drastically change hiking conditions. Life-threatening water flows in the narrows are evident from flood debris lodged between canyon walls at heights in excess of 50 feet above the stream.
Weather can be deceptive. Heavy rains 35 miles upstream can bring high water flows. As a general rule of thumb, water three-feet deep traveling three feet per second can knock a hiker down into murky waters.
Be cautious about hiking if thunderstorms are predicted in the Paria or Buckskin watersheds. The most dangerous areas within the canyons are the Paria Narrows and Buckskin Gulch, due to the lack of escape routes and high terraces. If a storm develops and a flash flood situation is created, get to the highest and safest point possible. Flash floods usually subside in 8 to 12 hours. Many times a flood can leave you stranded for several days due to treacherous footing, even with lower flows.
This link can provide information on stream flow for the Paria River.