Handbook of Nature-Study · Lesson CCX · Topic 32: The brook
210. How a brook drops its load
Goal: Your child can show with a jar that pebbles settle first, then sand, then mud, and can explain how deltas are built.
You'll need
- A large clear jar with a tight lid
- Water, a handful each of small pebbles, sand and mud
- A brook to visit (optional)
- A nature notebook and a pencil (colored pencils if you have them)
What's in the book
The lesson has a leading thought, a jar experiment with gravel, sand and mud from the brook, and ten observations on where the current is swiftest, soft and stony bottoms, a little dam, pools and stream deltas. A passage from L. H. Bailey about a boyhood brook follows.
The lesson, step by step
The leading thought
The brook carries its load only when it is flowing rapidly. As soon as the current is checked, it drops the larger stones and gravel first and then the finer sediment. It is thus that deltas are built up where streams empty into lakes and ponds.
The jar experiment
Fill the jar most of the way with water, add pebbles, sand and mud, put the lid on tight and shake hard. Set it down and watch.
Does the mud begin to settle while the water is in motion; that is, while it is being shaken?
Which settles first—the pebbles, the sand or the mud? Which settles on top—that is, which settles last?
Stir it a little
The next day, rock the jar gently.
if the water is disturbed even a little it becomes roily again, which means that as soon as the water is in motion it takes up its load.
At the brook (with an adult)
Look from the bank, or wade in a shallow, calm stretch at normal water level only:
Where is the current swiftest, in the middle or at the side of the stream?
Can you find a still pool in your brook that has not a soft, muddy bottom? Why is this?
A little dam
Place a dam across your brook where the bottom is stony, and note how soon it will have a soft mud bottom. Why is this?
Build a small dam of a few stones in a tiny side trickle or a rain rill, not across the whole brook, and take it apart when you leave.
Deltas
What becomes of the soil dropped by the brook as it enters a pond or lake? Do you know of any points of land extending out into a lake or pond where the stream enters it? What is a stream delta?
Nature notebook
Draw the jar with its layers labeled pebbles, sand and mud. Draw a stream entering a pond with a fan-shaped delta.
Answers to the book's problems
Pebbles settle first, then sand, and the fine mud last, on top. The current is swiftest in the middle; the sides are slower, so mud settles there and the bottom is softer. Behind a dam the water slows and drops its mud. A pool in the swift current may stay stony because the water never slows enough there. A delta is the fan of soil a stream drops where it slows down entering still water.
Worked out for this site from the scan. If a number in your copy differs, trust the book.