Detention vs. Retention Ponds: Stormwater Basics for Oregon Sites

Two Ponds, Two Very Different Jobs

When a commercial site adds pavement and rooftops, it sends more water off the property, faster, than the bare land ever did. Stormwater regulations exist to manage that change, and on larger sites the answer is often a pond. But "stormwater pond" covers two distinct structures — detention and retention — and they work in opposite ways.

Get the difference wrong in planning and you build the wrong basin. This guide lays out how each works, where each fits in Oregon, and what they cost to maintain. For the bigger drainage picture, start with our Oregon drainage guide.

Detention Pond: Hold and Release

A detention pond, also called a dry pond, is a temporary storage basin. During a storm it fills up, then slowly releases the water through a controlled outlet at a rate the downstream system can handle. Between storms it sits empty — hence "dry."

The whole point of a detention pond is flow control. It doesn't reduce the total volume of water leaving your site; it spreads that volume out over time so a downstream creek or storm pipe isn't overwhelmed. The key engineered feature is the release rate: the outlet is sized so the pond discharges no faster than the pre-development site did. That's usually the regulatory requirement.

Retention Pond: Hold and Keep

A retention pond, or wet pond, maintains a permanent pool of water — the dead pool — even between storms. Stormwater enters, mixes with the standing water, and the pond's permanent volume buffers and treats it before a controlled release.

Retention ponds do two things detention ponds don't. They provide water quality treatment as sediment and pollutants settle out in the standing water, and the permanent pool supports an aquatic ecosystem. They also lose some water to evaporation and infiltration over time, which modestly reduces total runoff volume. The trade-off is that a wet pond needs a reliable water source and more careful design to keep that pool healthy rather than stagnant.

Side by Side

Which One Does Your Oregon Site Need?

The answer is usually driven by the project's stormwater requirements rather than preference. A jurisdiction may require flow control, water quality treatment, or both. Detention handles flow control; retention handles flow control plus treatment. Many Oregon sites pair a treatment practice — a bioswale or water quality swale — with a detention basin to cover both bases efficiently, especially where land is tight.

Site conditions matter too. A high water table can make a dry detention pond hard to keep dry, nudging the design toward a wet pond. Poorly draining clay affects how an outlet and any infiltration are designed. These are the questions a stormwater engineer and your excavation contractor work through together. Sites like retail centers and office complexes often combine pond storage with surface collection — see our commercial parking lot drainage design guide for how the lot ties in.

The Release Rate Is Everything

Whichever pond you build, the outlet structure is the heart of it. It's engineered to meter water out at the allowed rate during a range of storm sizes — often using a combination of orifices and a weir or riser. Build the storage volume right but get the outlet wrong, and the pond either floods upstream by holding too much or fails its purpose by releasing too fast. This is precise, engineered work, not something to eyeball.

Maintenance: The Part Owners Forget

Stormwater ponds are infrastructure that needs upkeep, and neglected ponds fail inspections.

• Detention ponds need sediment removed from the bottom and forebay, vegetation managed, and the outlet kept clear of debris.
• Retention ponds need all of that plus pool health management — controlling algae, managing nutrients, and keeping the permanent pool from going stagnant.

Both need their embankments and outlet structures inspected, especially after major storms. Sediment accumulation is the slow killer; as it builds up, storage volume shrinks and the pond stops meeting its design.

Permits and Erosion Control

Building a pond means moving a lot of earth, which on larger sites triggers the Oregon DEQ 1200-C construction stormwater permit and a sediment control plan — see our DEQ 1200-C stormwater permit guide. During construction, the disturbed area has to be protected so the pond you're building doesn't become an erosion source itself; our erosion control excavation guide covers the BMPs.