Instead of asking students to just believe what you've said, why not ask them to verify your assertions? Students plot the points of the stream, perhaps map them with an equation generated by Excel or their calculator, and then use their algebra understanding to rearrange the equation into vertex form. Is the height of the stream where it should be?
The Activity: turtle-fountain-grid.pdf
For members we have an Excel sheet of the data, a Word doc, and solutions.
stream-graph.xls turtle-fountain-grid.doc turtle-fountain-solution.pdf
CCSS: HSA.SSE, HSA.CED, HSA.REI, HSF.LE