It is very important that students understand how water drains off the land to form streams and rivers that eventually (usually) flow to the ocean. Images like this will definitely help. CLICK HERE to see several other images and an impressive animation, created by Imgur user Fejetlenfej,a geographer and GIS analyst.
A map such as the one shown above can be very effective in helping explain the concept of watersheds (and divides). Click on the map to enlarge - and appreciate the detail. It might also serve as the focus of a "bell-ringer". For example, have students write out 3 quality questions about the map - stipulating that each question must begin with the word "why". Then in your discussion of the bell-ringer, ask if they would be able to use the map to mark the location of the Continental Divide (aka the Great Divide).