THE RULE OF V'S

 "Knowing which way a river flows is often the key to being able to label contours correctly! Look at what happens to the contour lines in this picture when they cross a river! " "They bend to form kind of a brown "V" shape! An upsidedown "V" in this case!"
 "You can see the "V"s on the contour map too! " "And the "V"s point upstream! "
 "By the way, Lou! Make sure not to confuse the "V"s the contours make (the brown "V"s) with the "V"s made by streams where they join (the blue "V"s)! It's the brown "V"s we want! Here, the streams flow towards the south!
 "The "V"s can be short or long, sharp or blunt, but they always point in the upstream direction! And the river flows the other way!"
 "So here we can see that where the D contour crosses the river it forms a "V" (shown in green) that points west! That means west is upstream! And the river flows away from the west towards the east!""And in that case, the higher area that the D contour wraps around and encloses must be to the west, not the east! So, the D contour lies outside the 700 foot contours! It must be the next lower contour! It must be the 600 foot contour!"
 "Excellent, Louis! Let's see if the idea has really stuck in the old beano! How about a problem, Louis? Go do the quiz!"