How did Jess get across the creek the first time in Bridge to Terabithia?

The first time Jess crosses the creek bed, it is early autumn, and the bed is dry.  He and Leslie have discovered an old rope hanging from a crab apple tree on the bank of the creek bed, and are taking turns swinging across the gully on it, leaning back and drinking in "the rich, clear color of the sky".  After awhile, Leslie comes up with the idea of creating a special place for the two of them, a secret country of which they would be the rulers, modeled after Narnia, the magic land described in the books by C.S. Lewis. Leslie decides that "the only way you can get in (to their secret land) is by swinging across on the "enchanted rope", so the first time Jess crosses the creek to get to Terabithia, he does it in this manner (Chapter 4).

It is early spring when Jess and Leslie first cross the creek to Terabithia when the creek is filled with water.  It has been raining steadily for days, and the creek is "an awesome sight...a roaring eight-foot-wide sea, sweeping before it great branches of trees, logs, and trash, swirling them about".  Jess is afraid and suggests that maybe they "ought to forget it" that day, but Leslie is insistent.  Leslie goes first, carrying P.T. in her jacket, then Jess jumps to grab the rope, and, "shutting his mind to the sound and sight of the water...(runs) back and then speed(s) forward".  With "the cold stream lapp(ing) his bare heels momentarily...he (swings) into the air above it and fall(s) awkwardly and land(s) on his bottom", safely within the boundaries of Terabithia (Chapter 9).

The first time Jess crosses the creek after Leslie dies, he hauls a large branch which has washed up during the storm and lays it bank to bank at the narrowest point of the gully.  The rope on which he and Leslie used to swing is gone, having broken when Leslie fell to her death.  Jess steps out on the branch, and finding it firm, he crosses it, "foot over foot, to the other side, grabbing the smaller branches which (grow) our from the main one toward the opposite bank to keep his balance".  It is the first time he has entered Terabithia without swinging into it over the creek, and he wonders momentarily if the magic of the land will still exist if he enters it in this alternate manner (Chapter 13).