If you're an average American, with an average American toilet, you probably flush away 35 gallons of water a day. That comes to almost 13,000 gallons a year. Multiply that by the number of people in your family and you quickly see that your toilet uses a tremendous amount of water every year.It probably uses up to 40 percent of all the water consumed in your home.

That's too much. It works out to around five gallons per flush -- more than is really needed for efficient flushing, and more than many sewage plants can handle.The result is wasted water, and pollution.

But there's something you can do about it. One of the cheapest and most effective steps is to install water dams in your toilet tank.

Water dams are simply plastic plates you can slip into place to limit the amount of water used for flushing. They don't impair flushing effeciency -- in fact, according to tests conducted by Popular Science, they can actually increase effeciency, providing more flushing power with less water.

These dams are made by two firms that I know of. Metropolitan Watersaving, Inc., 5130 MacArthur Blvd. NW, suite 106, 20016, makes dams called Little John. The other is Watersavers, Inc. Box 22326, Cleveland, Ohio 44122. They make the Moby Dike.

You may have heard that placing a couple of bricks or plastic containers full of water in your toilet tank will do the same job, and for less money. This is partly true; but the dams work better. They're adjustable, and won't create currents in the tank that can impair efficiency. Unlike bricks, they can't crumble and cause valves to stick open, wasting vast amounts of water. And plastic containers can move around and interfere with the action of moving parts inside the tank.

Another watersaving trick you can use: Slow the rate at which your toilet tank refills after a flush, by closing down on the tank valve. How will this save water? Well, as soon as you trip the flush lever, water to refill the tank starts to flow in. Meanwhile the tank is draining, so any water that flows in just flows right on out. By slowing the refill rate you cut down on the flow of water that flows into the tank while it is still draining.

There is one drawback: The tank now takes longer to fill, so you have to wait longer between flushes. But this is only a problem in a high-traffic bathroom.

Other ways to save? All the major toilet makers are now out with toilets designed to use less than the typical five gallons per-flush. Most do the job on three or so gallons. There is even a trend back to the old-style toilet, with the tank mounted high on the wall over the bowl. This lets the flushing water drop farther for more flushing power despite the reduced volume of water. 1978, Popular Science.