Danger River is the fourteenth mission in WorldBuilder.
The first thing you'll notice is a new enemy guarding the river. The Alligator is slower than Crabs, but you should still be careful, as Gators will also disassemble your units in one hit. But just like Crabs, Gators cannot cross rocky terrain.
Grab the plans for the Duck and the Dirtbuggy, then move the Duck on top of the stack of Wheel Bricks on the upper portion of the map and take it apart. Build a Dirtbuggy and move it near the Wheel Bricks on the other side of the riverbank, and take it apart. Build a Duck in the river, move it to the Wheel Bricks on the other side of the riverbank, then take it apart. Do it again, and use the parts to rebuild the Dirtbuggy.
Grab the plans for the Steamshovel, then get the Dirtbuggy back to the other side by taking it apart near the river, building a Duck in the river, taking the Duck apart, building another Duck in the same spot, taking that Duck apart, and building a Dirtbuggy on the other side. Grab all of the yellow bricks with the Dirtbuggy and drop them off in the same spot in the river near the wheel bricks, then take it apart. Build and take apart Ducks to get the yellow bricks across to the other side, then build a Dirtbuggy again. Pick up and drop off all of the yellow bricks past the rocky terrain, then take apart the Dirtbuggy and build a Steamshovel.