I Would guess that is the rocks in the river that is the first obstacle besides the river itself. I have a great fear of water. Chuckles. Long old story behind that one. .
The rocks are grey and ragged edged not worn and smooth like a normal river boulder would be. Look like something from an earthquake pulling them upright out of the bedrock.

or it could be the arroyo
The zizagging arroyo is a steep slick sided wash cut deep into the red clay earth and normally dry unless in the rainy season. With a flash flood danger that can come from 100 miles away without warning .

That is the only other obstacles I can think of before the wet quicksand at the bottom of the arroyo.