That's an Earth Science lab?! If so, I'm glad my kids go to school in NY. Teachers are welcome to use simulations, but all lab activities which count toward the required lab time must be hands-on. No simulations. (Although, that would be a great activity to do as a warm-up to a real lab doing the same thing in real-life.)
Anyway, to measure the width of the stream, drag the line across and line it up with the right side of the stream. (rightmost blue area) Click the ready to measure button .
Then move the slider back and forth a bit to the different colored blue areas and click on record. Do this until it says "I'll fill in the rest for you." Then click "done measuring." By this point, you will have learned next to nothing. (Go Chicago!)
On the next screen, you have to calculate area of a rectangle (width times depth) - type that in. (There's a calculator on the screen to use.) Then, multiply that answer by average velocity to get Discharge. Do that for 4 of them, inserting the data into the table (not the box for total discharge.) Then, press "verify calculations." Then, click "add remaining data" (which will show up after you verify calculations, assuming you can multiply two numbers on a calculator correctly. Then, click on "sum up discharge". And, finally, "go to next page."