Hilo Area Water Quality

The Wettest City in America

Hilo is the county seat of Hawaii Island and the wettest city in the United States, averaging over 130 inches of annual rainfall. The beaches here are fundamentally different from the dry, white-sand resorts on the Kohala Coast 60 miles to the northwest. Hilo’s coastline is volcanic rock, black sand, and tidepools fed by freshwater springs.

The constant rainfall and freshwater discharge create a challenging water quality environment. The Wailuku River, one of Hawaii’s largest rivers, empties into Hilo Bay. Smaller streams and springs run continuously through the porous lava rock along the Keaukaha coast where most swimming beaches are located. When it rains — which is most days — these water sources carry bacteria, nutrients, and sediment to the nearshore ocean.

Key contamination sources: Wailuku River discharge, freshwater springs along Keaukaha, constant rainfall, cesspools in the Hilo urban area, storm drains, and agricultural runoff from upland farms and ranches.

Source: NOAA climate normals; USGS stream data; Hawaii DOH CWB

Ranked by overall water quality risk. Dry weather baseline; all beaches see increased risk after rain.

Wet All Year, Wettest in Winter

Year-round: Hilo has no true dry season. Rain falls on approximately 270 days per year. Stream discharge is constant, not seasonal. Water quality at Hilo beaches is always influenced by freshwater input.

Winter (November–March): The wettest months, with some months exceeding 15 inches. Heavy Kona storms can cause river flooding. The Wailuku River can turn Hilo Bay brown for days. All beach water quality is at its worst during prolonged winter rain events.

Summer (June–August): The “driest” months in Hilo still see regular rainfall, but dry spells of 2–4 days are more common. These dry windows are when Hilo beaches are at their best for water quality. Plan beach time during these breaks in the rain.

Rain Is the Default, Not the Exception

In Hilo, the question is often not “has it rained?” but “has it stopped raining long enough?” The 72-hour rule is a useful guideline, but Hilo rarely goes 72 hours without rain.

Practical approach for Hilo: Focus on the intensity, not just the timing. Light trade wind showers are constant and have minimal impact. Heavy, sustained rainfall (1+ inch per hour) is what causes significant bacteria spikes. Wait at least 24–48 hours after heavy rain events.

Wailuku River: If the river is running high and brown, avoid Hilo Bay and Coconut Island entirely.

Visual assessment: In Hilo more than anywhere, look at the water. Clear water with good visibility suggests lower bacteria risk. Brown, murky water near stream outlets means avoid swimming.

Full after-rain swimming guide →

Hawaii DOH Clean Water Branch — Beach monitoring, advisories. eha-cloud.doh.hawaii.gov

USGS — Stream gauge data, rainfall records.

NOAA / NWS — Climate normals, weather patterns.