Direct answer: Detention ponds are designed to temporarily hold stormwater and release it slowly, drying down between storms to function as ephemeral wetlands or dry basins. Retention ponds hold a permanent pool of water year-round, releasing only excess storm volume. The two designs produce fundamentally different aquatic plant communities, maintenance requirements, and water quality benefits.
Hydrologic Distinctions
The critical difference is the presence of a permanent pool. A dry detention basin (also called extended detention basin) has an outlet at the lowest elevation so that water drains completely within 24–72 hours of a storm event. Between storms, the basin is dry or marshy. A wet retention pond has an outlet set above the basin floor, retaining a permanent pool of typically 3–8 feet depth with outlets discharging only the storm volume above the permanent water level.
Modern stormwater management practice has progressively favored wet retention ponds over dry detention because retention ponds provide superior water quality treatment via sedimentation, biological uptake, and chemical adsorption during the residence time. Detention basins primarily control flood peaks but provide modest water quality treatment. Many older detention basins are being retrofitted to retention designs as part of MS4 (municipal stormwater) permit compliance.
Aquatic Plant Communities
The two designs produce predictably different vegetation. Dry detention basins typically develop wet-meadow vegetation dominated by emergent species — sedges, rushes, smartweeds, and occasionally invasive species like phragmites or purple loosestrife. Maintenance includes periodic mowing to prevent woody encroachment and selective herbicide treatment for invasive emergents.
Wet retention ponds develop full lake-type aquatic plant communities with a littoral zone of emergent and submerged plants and an open-water zone. Without active management, retention ponds with elevated nutrient inputs (typical of urban watersheds) often develop dense beds of nuisance submerged species — hydrilla in the South, Eurasian watermilfoil across most of the country, duckweed mats in calm nutrient-rich waters. Stormwater retention pond aquatic weed management is one of the fastest-growing sectors of the U.S. aquatic vegetation management industry — see the stormwater basin management guide for design and operations recommendations.
Water Quality Performance
Wet retention ponds remove approximately 50–80% of total suspended solids, 30–60% of total phosphorus, and 20–40% of total nitrogen from stormwater under design conditions. The treatment performance depends on residence time (longer is better — design typically for at least 14-day residence), pool volume relative to drainage area, and pond geometry that minimizes short-circuiting between inlet and outlet. Pretreatment forebays catch coarse sediment and reduce the maintenance burden on the main pool.
Dry detention basins remove only 20–40% of TSS, 10–30% of phosphorus, and less than 20% of nitrogen — substantially lower performance than wet retention. The shorter residence time and absence of biological uptake account for the difference. Detention basins remain useful where space constraints, soils with high infiltration, or local groundwater conditions preclude wet retention design.
Maintenance Considerations
Wet retention ponds require ongoing maintenance: periodic sediment removal from the forebay (every 5–15 years), aquatic plant management for invasive species (typically annually), shoreline mowing and erosion repair, and outlet structure inspection. Annual maintenance costs commonly run $500–$3,000 per acre for residential retention ponds and substantially more for commercial or industrial sites. Dry detention basins cost less to maintain but provide less benefit — the choice is one of life-cycle cost vs water quality treatment.
HOA-owned retention ponds in particular often suffer maintenance neglect that produces severe aquatic weed problems within 5–10 years of construction. Establishing a formal maintenance plan and reserve fund at the time of construction — addressed in pond management plans — is the single most effective preventative for chronic stormwater pond weed problems.
Frequently Asked Questions
What is the main difference between detention and retention ponds?
Detention ponds drain completely between storms (dry between events); retention ponds hold a permanent pool of water year-round. This single difference drives fundamental differences in aquatic plant community, water quality treatment performance, and maintenance requirements.
Which provides better water quality treatment?
Wet retention ponds substantially outperform dry detention basins, typically removing 50–80% of suspended solids and 30–60% of phosphorus vs 20–40% and 10–30% respectively for detention. The permanent pool enables sedimentation, biological uptake, and chemical adsorption during the days to weeks of residence time.
Why do retention ponds get so weedy?
Urban and suburban watersheds typically deliver elevated nutrient loads to retention ponds — lawn fertilizer, pet waste, and atmospheric deposition. Combined with shallow water depths and long residence times, this creates ideal conditions for nuisance aquatic plant growth. Without active management, most retention ponds develop dense weed beds within 3–7 years of construction.
Can I stock fish in a retention pond?
Yes, and stocking can help with vegetation management. Triploid grass carp (where state-permitted) can effectively suppress some submerged plant species. Bass and bluegill populations provide recreational value and contribute to insect and aquatic vegetation balance. State permits typically govern fish stocking in stormwater ponds — check local regulations.
How often does a retention pond need dredging?
Forebay sediment removal is typically needed every 5–15 years; full main-pool dredging may be needed every 25–50 years. Frequency depends on watershed sediment yield, pretreatment effectiveness, and design depth. Sediment depth surveys every 3–5 years inform maintenance planning.
Can I convert a dry detention basin to a wet retention pond?
Often yes, depending on site conditions. Retrofit requires modifying the outlet structure to retain a permanent pool, ensuring adequate watershed flow to maintain water levels, possibly excavating to increase pool volume, and possibly adding liner material in highly permeable soils. Retrofit costs are commonly $20,000–$200,000 depending on site complexity but produce substantial water quality benefit.
Are detention or retention ponds better for mosquito control?
Wet retention ponds with healthy fish populations and circulation typically have fewer mosquito issues than dry detention basins or stagnant retention ponds. The key for either design is preventing prolonged stagnant shallow water where mosquitoes breed. Regular maintenance, aeration, and balanced fish populations are the most effective preventatives.
References
- U.S. EPA (2009). Stormwater Wet Pond and Wetland Management Guidebook. EPA 833-B-09-001.
- Hunt, W.F., et al. (2011). Plant selection for bioretention systems and stormwater treatment practices. SpringerBriefs in Water Science and Technology.
- Center for Watershed Protection (2007). National Pollutant Removal Performance Database, Version 3.
- Schueler, T.R. (1987). Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs. Metropolitan Washington Council of Governments.
- Walker, W.W. (1987). Phosphorus removal by urban runoff detention basins. Lake and Reservoir Management, 3(1), 314–326.
Ten-Year Lake Management Plan: Lake Wingra, WI
Lake Wingra, a 342-acre urban lake in Madison, WI, developed a comprehensive 10-year management plan coordinating the City of Madison, University of Wisconsin, and adjacent neighborhood associations. The plan addressed Eurasian watermilfoil, curly-leaf pondweed, and purple loosestrife through an integrated approach including targeted herbicide treatment, mechanical harvesting, native plant restoration, and public education.
Key outcome: The structured multi-agency planning process secured consistent funding across multiple budget cycles, a key advantage over ad hoc management. Native plant restoration efforts showed measurable progress in designated restoration zones within three years of initiation.
Running a golf course with three retention ponds means constant weed pressure. The prevention and best management practices guide gave us a systematic approach that replaced our reactive spray schedule.
Paul Esteban Golf Course Superintendent, SC · Myrtle Beach areaAs a lakefront property owner I was completely lost until I found AquaticWeed.org. The permit guidance alone saved me from making costly, potentially illegal treatment mistakes.
Gerald Renfrew Lakefront Landowner, WI · Vilas County