What river forms a natural barrier between the United States and Mexico?

all fossil fuels are nonrenewable, so think coal, gasoline, things that we will eventually run out of. all fossil fuels are nonrenewable resources, but not all nonrenewable recourses are fossil fuels. these resources are not being renewed as fast as we are using them. i hope this considering there weren't any answer choices.

answerpeople organize large projects such as levee-building;