How to build a backyard waterfall and stream into a pond

A backyard stream starts at a waterfall filter set at or slightly below grade, runs 2 to 4 feet wide and 6 inches to 1 foot deep, and loses its height in a few short waterfalls instead of a long smooth ramp. Underlayment goes down first and liner over it, and wherever two sheets meet, the upstream sheet laps over the downstream one so water runs across the joint instead of into it. Foam fills the gaps under and between the spill stones so the water stays on top of the rock. No equipment maker publishes a slope in inches per foot, and the two figures that circulate differ by a factor of six.

What the manufacturers say

Aquascape's Signature Series BioFalls manual is the most detailed manufacturer document for this job. The filter "should be set at, or slightly below, the grade of the yard," on compacted soil, and the manual warns against "creating a 'volcanic look' by trying to raise the BioFalls Filter in a flat backyard." The box should be "level from side-to-side and tilt forward at 1/4 of a bubble on a 2' level," which "will make sure the water covers the entire spillway."

For the stream, the manual says "The typical width of a stream should be between 2-4 feet wide," and to "Excavate the stream to a depth of 6 inches to 1 foot." On a slope "you will need to create a few waterfalls in it," and "To hold the water back when the pump is shut off, you need to build a dam at each waterfall." Boulders go into pockets dug "a few inches deeper" so they sit down in the streambed. On joining sheets: "Simply overlap the stream liner over the top liner. As long as you have a 6" waterfall or higher, you will not need to seam the liners together." The rocks are set on foam so water is "diverted around and over the rocks instead of underneath them."

The manual also covers something most guides skip. Foam joints seep when the pump is off for a long time, and water creeps around a thick spill stone, "resulting in water loss equal to the thickness of the stone," so it recommends a stone a half inch to 1 inch thick, or recessing a thicker one. And it ends with "Do not trim the liner until the waterfall is running. Prematurely trimming the liner may cause leaks!"

The Pond Guy's stream guide agrees that "you do not need to seam the stream liner to the pond liner. Simply place the stream liner over the pond liner so water can flow properly." It sets the waterfall box "level side to side but tipped about an inch forward," a different instruction from Aquascape's quarter bubble.

For underlayment, Firestone's PondGard installation guidelines recommend a geotextile between soil and liner and call it "an absolute necessity on slopes where deposition of an additional support layer is often difficult." It says "geotextiles are to be installed with an overlap of at least 12 inches (300 mm)." Firestone's seam rules are written for taped seams and do not address the unseamed overlap that Aquascape and The Pond Guy describe.

What contractors and experienced owners do

In TEAM Aquascape's Build Your Own WATERFALL: Pro Tips Revealed, the crew builds every falls the same way: "big rock on one side big rock on the other side something in between." They "dig almost back pitch the pooling area that the waterfall dumps into," so water pools before the next drop instead of pushing gravel off the liner, and they cut and overlap the liner to make the stream turn.

In WATERFALL Built In 2 HOURS!, the TEAM Aquascape crew rebuilds a family's first attempt and names the usual error: "the biggest mistake when people build waterfalls is to make them high and it looks like a volcano spilling lava." At the spillway they "foam here then we're gonna put the underlayment down then we're gonna foam again," and they admit "this is only about a 90 percent seal," which is enough because most of the water still goes over the lip.

On the Garden Pond Forum, owners report the unseamed overlap working. Moderator Lisak1 writes that "the bottom liner needs to come up under the top liner high enough that you are confident it won't slip down." Administrator addy1 has a stream "around 85 feet long at every turn and where it drops into ponds I just did overlap, no seams no glue, it worked well." The dissent comes from sissy, who found that "plants and algae can grow under the 2 liners and lift them up," even with 3 to 4 feet of overlap.

Shane Stefek, writing in POND Trade Magazine, gives the only flow figures in these sources written for streams: "A creek or stream may require 250 to 500 gph per inch of width," while a babbling brook needs 100 to 150 gph per inch at the widest point. He also writes "At minimum you need 2 feet of rise for every 20 feet of land to create enough slope."

Where the sources differ and why it matters

The slope figure seen most often traces back to one magazine project. Family Handyman wrote "Not much slope is needed (minimum two inch drop per 10 feet of stream)," and Koi Pros repeats it almost word for word, along with the same width and depth. Stefek's minimum is six times steeper, and neither Aquascape nor The Pond Guy gives a number at all.

The two numbers describe different streams. Four inches over 20 feet makes a slow, quiet run, and Family Handyman pairs it with drops of "two to four inches" for a babbling sound and "10 inch and greater" drops to cover street noise. Twenty-four inches over 20 feet looks natural only if most of the fall happens at a few waterfalls with pools between them, which is the Aquascape method.

Overlap length differs too. Aquascape asks only for a 6 inch waterfall at the joint, while Family Handyman says "the top liner overlaps the liner underneath it by 1-1/2- to 2-ft." Going with the longer overlap costs only a little more liner.

What to do

1. Call 811 before digging, as The Pond Guy advises. Face the falls toward where people sit, use the slope the yard already has, and set the filter at or just below grade on compacted soil.
2. Choose the flow first, because the spillway width and pump follow from it. Our pump sizing article covers gallons per hour per inch of spillway and head loss.
3. Dig the stream 2 to 4 feet wide and 6 inches to 1 foot deep, with a flat or back-pitched pool below each drop and deeper pockets where boulders will sit.
4. Lay underlayment, overlapping its pieces by at least 12 inches, then liner. Work from the pond upward so each upstream sheet laps over the one below, ending in a drop of 6 inches or more. Leave slack at the base of each falls so the rocks do not stretch the liner.
5. Set two frame boulders at each falls with the spill stone between them. Use thin spill stones or recess thick ones, and foam under and between them until the water runs only over the top.
6. Run the water before trimming anything. Watch the edges and the spillway seal for wet soil, since that is where stream leaks usually show up, then trim the liner, leaving several inches above the water line.

Plan for the water the stream holds. Family Handyman figures "five gallons per linear foot of flowing stream (2-1/2-ft. wide x 3-in. deep)," so a 20 foot stream holds about 100 gallons. On a 3,000 gallon pond with about 200 square feet of surface, that is about 0.8 inch of pond level by our arithmetic, which the pond loses when the pump starts and regains when it stops unless dams hold the water in the stream. Stream rock adds to the pond's total; one Aquascape distributor's rule is three quarters of a ton per 10 feet of stream, and our rock tonnage article compares it with the other rules.

A meandering stream needs a wider sheet, or several pieces lapped at the turns the way the TEAM Aquascape crew does it.

Laying it out in PondTools

The PondTools liner calculator (also standalone) takes the longest span, the widest span at right angles to it, the maximum depth, the buried overhang and a fold allowance. Enter the stream's length, width and depth, then add the drop height and the lap onto the pond liner as in the table. The PondTools pump calculator (also standalone) takes pond volume, lift to the top of the falls, pipe size and length, a flow to check and two points off a pump's curve, and says whether that pump delivers the flow. In the PondTools app you trace the pond and stream from a satellite or drone photo and get their surface area and gallons before anything is dug. The PondTools studio renders the pond and falls in 3D at any time of day, so a client can see the waterfall from the deck before anyone digs.

Sources

• Signature Series BioFalls Filter Instructions and Maintenance, Aquascape (hosted by The Pond Guy)
• PondGard Lining Systems Installation Guidelines, Firestone
• How to Build a Backyard Stream, The Pond Guy
• Build a Backyard Waterfall and Stream, Family Handyman
• A Few Tips and Tricks on Backyard Waterfall Installation, Koi Pros
• A How-To Guide: Building a Perfect Stream, Shane Stefek, POND Trade Magazine
• Planning Pond Calculations, Meyer Aquascapes
• Build Your Own WATERFALL: Pro Tips Revealed, TEAM Aquascape (YouTube)
• WATERFALL Built In 2 HOURS!, TEAM Aquascape (YouTube)
• Liner question, Garden Pond Forum