When do I need a stormwater permit for construction?

When the project disturbs one acre or more, or is part of a larger common plan of development disturbing one acre. You must obtain coverage under the IPDES Construction General Permit before starting earthwork.

Can my city require a SWPPP for smaller sites?

Yes. Cities operating under MS4 permits often require erosion control plans for sites under one acre. Local rules can be stricter than IPDES but cannot waive state requirements.

What happens if I discharge without a permit?

DEQ may issue compliance orders and civil penalties. The federal Clean Water Act authorizes additional fines up to $25,000 per day for unpermitted discharges to waters of the United States.

Idaho Stormwater Management Law: Statewide Environmental Rules Rules (2026)

Key Facts

Trigger area: 1+ acres disturbed
Permit: IPDES CGP IDR100000
Authority: Idaho DEQ
Statute: Idaho Code 39-3601
SWPPP required: Yes, on-site

Full Breakdown

EPA delegated NPDES authority to Idaho in 2018, and the Department of Environmental Quality (DEQ) now administers the IPDES program under Idaho Code 39-3601 et seq. Construction operators that disturb one or more acres, or are part of a common plan disturbing one acre, must obtain coverage under the IPDES Construction General Permit (CGP IDR100000). The CGP requires a Stormwater Pollution Prevention Plan (SWPPP), best management practices for sediment and erosion control, regular inspections by qualified personnel, and post-construction stabilization. Industrial facilities and certain Municipal Separate Storm Sewer System (MS4) operators require separate IPDES permits. Cities may adopt stricter local stormwater ordinances under their MS4 permits but cannot relax IPDES discharge limits.

Violations & Penalties

Discharging without a required IPDES permit can result in DEQ enforcement orders, civil penalties up to $10,000 per day, and federal Clean Water Act liability for repeat violations.

Idaho IPDES Stormwater Permitting Applies Statewide

Key Facts

Full Breakdown

Violations & Penalties

Frequently Asked Questions

Sources

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