The rivers we paddle have a very wide variety of features and topography.  Some of the challenges you might face are dense brush, boulders in the way, very fast water, ledges, shallow and slippery slides, river bends, etc.  Take a few minutes to assess the situation.  Don't fall into the tunnel vision trap, look upstream and downstream as well.  In one of the SWR classes I took, students resorted to paddle wading to cross a fairly wide stretch of river.  I ran upstream and quickly swam across.  They were still wading 20 minutes after I had crossed.  My flat water swim was much safer as well.  In a recent class, we had to exercise the line crossing exercise in a narrower area.  I thought I was giving the students a challenge by drawing an imaginary line 30' back from the stream edge.  One bright student said he could toss the rope the roughly 90' distance.  Not convinced I said go for it.  He simply tied together a couple of 1/4" lines, fed out one of the bags, and then threw the first bag overhand in a bulls eye to the person on the other side.  This is an ingenious adaptation of the messenger line approach used by fire departments - simply brilliant.