The Saddlecomb Ridge Map: Finding the Divide

Reading the Land at Saddlecomb Ridge

Dalia Renk draws maps for the water department in Nunfield, a fictional town. Her newest map covers Saddlecomb Ridge, the long hill east of town. The map shows no streets and no houses. It shows how high the land is.

Every square on the map holds one number. That number tells how many feet the spot sits above sea level. A square marked 800 is high ground. A square marked 410 is low ground. Two squares side by side tell you which way the land tilts.

Water follows one plain rule. It runs downhill. Rain that lands on a slope moves toward the lower ground next to it, and it keeps moving until it reaches a stream.

That rule is why a ridge divides the rain that falls on it. A line of high squares runs along the top of Saddlecomb Ridge. Rain that falls west of that line runs west and ends up in the Wynnow River. Rain that falls east of the line runs east and ends up in the Fernlow River. Mapmakers call that high line a divide. All the land that drains into one river is that river's watershed, so the divide is the edge between two watersheds.

The divide matters to Nunfield because the town drinks from a reservoir on the Wynnow River, at the low west edge of the map. Anything that washes off the west slope can reach that reservoir. Anything that washes off the east slope goes the other way, into the Fernlow River, and never comes near the town's water.

Dalia marked three working sites on her map. The county keeps a pile of road salt in a shed at square B5. A gravel quarry digs at square C2. A dairy pasture sits at square E3. In a hard rain, water leaves all three sites carrying something with it: salty water, gray mud, and manure.

Saddlecomb Ridge elevations in feet above sea level. Columns run west to east, and rows run north to south. The Wynnow River runs down column 1, the Fernlow River runs down column 6, and the town reservoir sits at square E1.

Dalia walks the ditches after big storms to check her own work. When the water goes somewhere the numbers did not predict, she fixes the map.

1. The passage says every square on the map holds one number. What does that number tell you about the spot?

2. The passage names the water the town of Nunfield drinks. Where does it come from, and which edge of the map is it on?

3. Row A of the map reads 470, 560, 660, 800, 640, and 500 feet from west to east. Which square in row A is the highest ground, and how high is it?

4. Rain falls on square C5, which sits at 600 feet. Its neighbors are C4 at 690 feet and C6 at 480 feet. Which neighbor does the rain run toward, and why?

5. Find the highest square in each row from A to E and write down its column number. What pattern do those five column numbers show as you move from the north end of the ridge to the south end?

6. The salt shed sits at square B5, at 620 feet. The highest square in row B is B4, at 790 feet, and B6 at the east edge is 490 feet. When rain dissolves some of the salt pile, does the salty water run toward the Wynnow River or the Fernlow River?

7. A dairy pasture sits at square E3, at 700 feet. In row E, E2 is 740 feet, E4 is 650 feet, E5 is 545 feet, and E6 is 455 feet. The Wynnow River is two squares west of the pasture, and the Fernlow River is three squares east. Which river does rain running off the pasture reach? Use the row E numbers to explain.

8. Column 1 reads 470, 455, 440, 425, and 410 feet from row A in the north down to row E in the south. Column 6 reads 500, 490, 480, 470, and 455 feet over those same rows. What do the two columns tell you about the direction both rivers flow?