The short answer
- A detention pond (dry pond) temporarily holds stormwater and releases it slowly through an outlet, then drains dry between storms. Its main job is reducing peak flow and downstream flooding.
- A retention pond (wet pond) keeps a permanent pool of water. Incoming runoff displaces water that has already been treated, and the pool provides settling and biological treatment in addition to peak control.
Side-by-side comparison
| Detention (dry) pond | Retention (wet) pond | |
|---|---|---|
| Water between storms | Dry | Permanent pool |
| Main purpose | Peak flow control | Peak control plus water quality treatment |
| Pollutant removal | Limited, unless designed as extended detention | Better settling and nutrient uptake |
| Space | Can double as a field or open space | Pool takes permanent space |
| Common concerns | Clogged outlets, standing water, mowing | Algae, sediment buildup, mosquitoes, safety around water |
| Typical use | Commercial sites, subdivisions with flood control needs | Larger developments, sites needing treatment, amenity ponds |
Extended detention ponds hold water longer (often a day or more) to let more sediment settle, closing part of the treatment gap.
Other stormwater basins
- Infiltration basin: holds runoff until it soaks into the ground. Reduces volume and recharges groundwater, but needs permeable soils and pretreatment to avoid clogging.
- Sediment basin: a temporary construction basin that captures sediment during grading. Under the EPA CGP it must store the 2-year, 24-hour storm runoff or 3,600 cubic feet per acre drained. Many sediment basins are later converted to permanent ponds. Try the sediment basin calculator.
- Underground detention: pipes, chambers or vaults beneath parking lots that work like a detention pond where surface space is limited.
- Bioretention: a shallow planted basin with engineered soil that filters and often infiltrates runoff. See post-construction stormwater.
What is a catch basin?
A catch basin is a storm drain inlet structure with a sump below the outlet pipe that traps sediment, trash and debris before water continues into the storm sewer. Catch basins need regular cleaning, typically with a vacuum truck, or the sump fills and pollutants pass straight through. During construction, catch basins and other inlets need inlet protection.
Maintenance
- Inspect after major storms and on a set schedule, often annually at minimum under local maintenance agreements.
- Keep outlets clear of debris, trash and vegetation.
- Remove sediment from forebays and the basin before it reduces design capacity.
- Manage vegetation: mow dry ponds, control woody growth on embankments, and manage invasive plants.
- Check embankments and spillways for erosion, seepage and burrows.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on the Clean Water Act, EPA NPDES rules and the EPA Construction General Permit. Your governing permit (EPA, state, tribal or local) controls. This is not legal advice, not EPA-approved, and not a California QSP/QSD or other state credential, and it does not issue a certificate. Nothing you enter on this page is saved or sent.