After Easter Sunday, when Leslie went to church with Jesse and his family, the rains start up again, which makes the ground very muddy:
"On Easter Monday the rain began again in earnest. It was as though the elements were conspiring to ruin their short week of freedom. Jess and Leslie sat cross-legged on the porch at the Burkes', watching the wheels of a passing truck shoot huge sprays of muddy water to its rear."
This is important to note because the rains affect whether or not Jesse and Leslie can play safely in Terabithia. And in fact, it was raining a lot before Easter, it was just cloudy on Easter Sunday, and now it's raining again. But the kids are determined to go out and play in their special spot, and this is what they see:
"When they got to the bank of the creek, they stopped. It was an awesome sight. Like in The Ten Commandments on TV when the water came rushing into the dry path Moses had made and swept all the Egyptians away, the long dry bed of the creek was a roaring eight-foot-wide sea, sweeping before it great branches of trees, logs, and trash, swirling them about..."
At this point, readers should notice that it's very dangerous to play around a creek that's this active due to the rains, and it's also very dangerous to swing across the creek on the rope. The author is laying down some foreshadowing here, giving us a glimpse of what might happen if someone wasn't careful while playing around the creek. As the chapter continues, we learn that the rains keep on for at least two more days, and that the creek continues to rise. Yet the kids keep on pushing the boundaries, swinging across the creek on the rope and going to their preferred spot to play. It's extremely dangerous, and all that tension and description of the bad weather are preparing us for what happens to Leslie when she goes to Terabithia alone.