Last week we had a big thunderstorm pass over the house during the evening, and my two boys (ages 10 and 6) put on their raincoats and went out into the front yard to see what was going on. They love to send leaf boats down the edge of the street when it rains. When I came out to check on them, they had piled up some landscaping gravel in the gutter to make a dam, causing a large puddle of murky rainwater to collect in the street near our driveway. The two of them were really excited about their civil engineering: “Look, Dad! We made a dam! But our lake keeps rising and flowing around the dam, so we need more gravel!” I grinned. This arms race continued until their dam started to leak through the gravel, kind of like the turbines at the bottom of Glen Canyon Dam. “What? You guys aren’t generating electricity?”
They noticed that the water coming into their little lake was murky with silt, while the water seeping out through the gravel was clear and clean. “Yep, you guys have built a sediment trap“, I explained. “The fast-flowing river water carries dirt with it, but in the calm lake water the dirt drops out to the bottom. That’s why it’s called a sediment trap, because the river sediment can’t get past the lake.” “Wow! We built a sediment trap!” they exulted. “Sediment trapping is a big problem for silty rivers like the Colorado and the Nile“, I elaborated. “The Glen Canyon Dam and the Aswan High Dam trap the river silt and prevent it from flowing downstream. That’s why the Grand Canyon and the Nile Delta are losing their sand.” [Yes, sometimes I talk in html.] The boys were thrilled to hear the world-wide implications of their little experiment.
After the rain we put the gravel back around the mailbox and washed our hands. And I thought to myself, What a great neighborhood / country / life I have! My kids can play safely in the yard, forget their video games, get dirty, have fun, and learn about fluid mechanics all at the same time!