Harvesting rain

Water-harvesting earthworks: berms, swales and basins

Before you buy a tank or a pump, look at the ground. A few inches of reshaped soil can hold a storm’s runoff long enough for it to soak in, with no moving parts and nothing to maintain but mulch.

Updated 26 September 2026

What earthworks do

Most yards shed water because they were graded to. Roofs, driveways and compacted lawns send runoff to the street as fast as it falls, and the soil a few feet away stays dry. An earthwork reverses that: it slows water down, spreads it out and gives it a place to sit while it infiltrates. The payoff is less erosion, less standing water where you do not want it, deeper-rooted plants that need less irrigation, and less runoff leaving the lot during a storm.

Every technique on this page is a variation on two shapes: a raised mound that holds water back, and a low area that holds water in. They are usually built together, because the soil dug out of one becomes the other.

Before you dig

• Test the soil. Dig a hole about 12 inches deep, fill it with water, let it drain, then fill it again and time the drop. Around half an inch an hour or better is workable for an infiltrating feature. Much slower than that and water will sit for days; amend the soil or plan for an overflow that carries the excess away.
• Keep water away from buildings. Hold any feature that collects water at least 10 feet from a foundation, and further on a slope that falls toward the house.
• Mind what is underground. Call 811 before digging so utilities are marked, and keep infiltration features off septic drain fields.
• Do not hand your water to the neighbors. Redirecting runoff onto an adjoining property can create liability. Every feature needs an overflow that ends somewhere safe on your own lot.
• Size it. The runoff calculator turns catchment area and soil type into the volume a basin needs to hold.

Berms

A berm is a mound of soil, usually longer than it is tall. On its own it adds height to a flat garden or a dry, walkable path through a wet one. Paired with a basin, it becomes the downhill wall that holds water in place.

• Build it from the soil excavated for the basin beside it, and compact it lightly in layers.
• Keep the top level so water does not concentrate at one low point and cut through.
• Give it a deliberate spillway: a low, rock-lined notch where overflow leaves during a big storm, instead of wherever the berm happens to be weakest.
• Plant or mulch it right away. Bare soil on a berm erodes in the first heavy rain.

Basins and swales

A basin is a bowl-shaped low area; a swale is a long, shallow channel. Both collect runoff and let it percolate into the soil where plants can use it. A rain garden is simply a basin that has been sized, planted and given an inlet and an overflow.

• Put them where the water is. Place a basin uphill of trees or beds you want to water, or downhill of a spot where water pools and should not.
• Level the bottom. A sloped floor drains to one end and the rest of the basin does nothing.
• Build swales on contour. A swale that follows a line of equal elevation, across the slope rather than down it, spreads water evenly along its length. Mark the contour with a string line and a line level, or a simple A-frame level, before digging.
• Plan for debris. Leaves and sediment collect in the low point. Clearing them once a year keeps the holding capacity you designed for.

Swales can be planted, mulched with wood chips, or finished with rounded stone as a dry creek bed that reads as landscaping for eleven months of the year.

Diversion swales

A diversion swale is built with a slight fall, rather than dead level, so it moves water instead of holding it. Use one to carry runoff from a downspout or a driveway edge to a basin elsewhere, or to pass overflow from one earthwork to the next so a series of small features handles a storm that would overwhelm one large one. Keep the grade gentle, around 1 percent, and plant or line it so the flow does not scour the channel.

Infiltration basins

An infiltration basin is a shallow, mulched depression built specifically to soak water in, with no expectation of holding it for long. A ring-shaped basin dug around the drip line of a tree delivers runoff to the roots that need it; a strip beside a driveway catches the sheet of water coming off the pavement. The mulch keeps the surface open so water enters rather than crusting over and running off.

French drains

A French drain is a gravel-filled trench, usually with a perforated pipe, that intercepts water below the surface and carries it somewhere else. Where a basin works with water on the surface, a French drain works with water in the ground, which makes it the tool for soggy lawns, wet slopes and water tracking toward a foundation. It needs a place to discharge: a downslope outlet, a dry well, or a sump. The French drain guide covers how they are built, and the French drain cost calculator prices one by length, depth and outlet.

Terraces

On a steep slope, terraces cut the hill into a series of level steps. Each flat shelf catches water that would otherwise run straight down, and each riser, whether stone, timber or a planted bank, slows what gets past. Low terraces with a planted edge are a reasonable weekend project. Walls taller than about 3 to 4 feet, or any wall holding back a slope above a house, driveway or road, are structures: many jurisdictions require a permit and an engineer for them.

Check dams

Where runoff already follows a channel, such as a gully, a drainage ditch or a swale on a slope, a series of small check dams turns one fast flow into a staircase of slow pools. Build them from rock, logs or fiber rolls across the channel, lower in the middle than at the edges so water spills over the center instead of cutting around the ends. Space them so the base of each dam sits at about the height of the crest of the one below it, and the channel fills with sediment and plants instead of eroding deeper.

When earthworks are not enough

Earthworks manage water on the surface of your own lot. If water is coming through a basement wall, start with foundation and basement water. If a lake, canal or bay is eroding the bank, soil shaping will not hold it; see seawalls, bulkheads and riprap.

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