Strainers are trees in the water that allow water to pass through, but not people or boats. Use good boat control to manuever around them. Don't stand or wade in swift water greater than knee deep. Your feet can become trapped between rocks and the current can push you over and hold you underwater. Cold water kills.
Early spring is the most dangerous time.
Don't be fooled by 75 degree temperature. Don't paddle in a lightning storm. Get off the water and wait it out. Don't paddle in situations that are above your skill level. Take classes, read books and magazines, watch videos and paddle with more experienced paddlers until your skills improve.