Kailua, Lanikai, Waimanalo, Bellows, and Kualoa — the trade wind coast where rainfall makes all the difference.
Loading current advisory count…
Source: Hawaii DOH Clean Water Branch · Updated automatically
The Windward Coast stretches from Makapuu Point north through Waimanalo, Kailua, Kaneohe, and up to Kualoa and the North Shore border. This is the wetter side of Oahu — the Ko’olau Range forces moisture-laden trade winds upward, dropping 40 to 60 inches of rain annually. By comparison, Waikiki sees about 20 inches.
That rainfall is the dominant factor in water quality. Multiple streams descend from the Ko’olau valleys, and after heavy rain they carry sediment, nutrients, and bacteria into nearshore waters. The good news: when it has not rained recently, windward beaches are among the most beautiful and generally clean on Oahu, thanks to strong trade wind flushing and open ocean exposure.
Key contamination sources: Stream runoff from Ko’olau valleys, Kawainui Marsh drainage into Kailua Bay, Kaneohe Stream system, aging cesspools in residential areas, and stormwater from the H-3 corridor.
Source: NOAA climate normals; USGS Hawaii stream gauge network; Hawaii DOH CWB
Ranked by overall water quality risk. Rating considers stream proximity, rainfall impact, historical bacteria data, and DOH monitoring results.
Summer (May–September): Trade winds are steady but rainfall is lighter. Stream flows are lower, and bacteria levels tend to be at their annual minimum. This is the best window for Windward Coast water quality.
Winter (October–March): Kona storms and increased frontal systems bring heavier rainfall. Stream discharge increases significantly. Kailua Bay and Kaneohe Bay see more brown water events. The trade wind pattern can shift to southerly Kona winds, which disrupts the normal flushing that keeps windward beaches clean.
Year-round: Unlike the North Shore, the Windward Coast does not have seasonal surf danger. Trade wind swells are generally manageable. The primary seasonal variable is rainfall and its effect on water quality, not wave size.
The Windward Coast is where the 72-hour post-rain waiting period matters most on Oahu. This is the wettest part of the island, and stream discharge after storms can be dramatic — brown water plumes visible from the Ko’olau ridgeline spreading into Kailua Bay and Kaneohe Bay.
Specific drainage patterns:
Kailua Beach south end: Kawainui Canal discharges marsh runoff here. Avoid the south end of Kailua Beach for 48–72 hours after heavy rain. The north end recovers faster.
Kaneohe Bay area: Multiple streams (Kaneohe, Waiahole, Waikane) feed into the bay. After significant rain, the entire bay can show elevated bacteria. Kualoa is at the north end and gets affected last.
Lanikai and Waimanalo: These beaches recover fastest because they lack major stream inputs. Even here, wait at least 24 hours after heavy rain before swimming.
Explore the Windward Coast with these top-rated tours and activities.
Tours listed via Viator and GetYourGuide. Safe to Swim Hawaii may earn a commission if you book, at no extra cost to you.
Free alerts when advisories change on Oahu beaches.
Hawaii DOH Clean Water Branch — Beach monitoring, advisories. eha-cloud.doh.hawaii.gov
USGS — Stream gauge data for Ko’olau streams, rainfall records.
NOAA / NWS — Climate normals, rainfall data, trade wind patterns.
City & County of Honolulu — Stormwater management, Kawainui Marsh drainage data.
⚠️ Important Disclaimer
Safe to Swim Hawaii is an independent project — not affiliated with any government agency. Water quality assessments are based on publicly available data including DOH monitoring, USGS stream gauges, and historical bacteria records. They are not real-time measurements. Conditions change rapidly, especially after rain.
Always verify with the Hawaii DOH Clean Water Branch before entering the water.
When in doubt, don’t go out.