Volcano Watch — The Wailuku River: mo`o and lava

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The Wailuku River is an important landmark to geologists, because it marks the approximate boundary between the lava flows of Mauna Kea and Mauna Loa.

The Wailuku River is an important landmark to geologists, because it marks the approximate boundary between the lava flows of Mauna Kea and Mauna Loa. It is the state's longest river and the southernmost that carries water all year. An average of 1 million cubic meters (275 million gallons) of water flows through the Wailuku to Hilo daily, generating some of the electrical power used on the Big Island. During intense storms, the discharge can be more than 20 times greater. On average, the Wailuku transports approximately 10 tons of suspended sediment into Hilo Bay each day. Hawaiians were impressed by the Wailuku and wove a dramatic tale around several interesting geologic features within the river. Hina, the mother of Maui, lived in the cave beneath Rainbow Falls, concealed by the mist of the falls. Each day she beat and dried her kapa in the sun. A large lizard named Mo`o Kuna occasionally disrupted this tranquil scene by sending torrents of water and boulders over the falls.

During a particularly intense storm, Mo`o Kuna moved a huge boulder over the falls and into the river, where it fit perfectly and prevented water from flowing farther. Water level beneath the falls began to rise. Hina, realizing her danger, signaled her son. With two powerful strokes, he paddled his canoe from Maui to the mouth of the Wailuku. He rushed upstream and split the damming boulder with a single blow, thereby saving his mother.

By this time, Mo`o Kuna had fled upstream. Maui found him hiding in a hole beneath the river. He tried to spear Mo`o Kuna, but the lizard escaped and rushed downstream. Finally, the mo`o found deep hiding holes and thought he was safe. Maui again found him and called upon Pele to send lava into the river to drive out the mo`o. When the lava reached the hiding hole of the mo`o, the water boiled, sending steam high into the air. The mo`o was killed; Maui carried its carcass downstream and threw it over Rainbow Falls for his mother to see.

Maui's canoe can be seen in the river just above the Pu`ueo bridge. It is part of a lava channel within the river, many other sections of which are also visible. People knowledgeable about Rainbow Falls can look into its plunge pool and see the remains of Mo`o Kuna.

The last hiding place of the mo`o is at Boiling Pots in Wailuku River State Park. Deep potholes eroded into old lava flows and probably connected below water level can become turbulent during winter storms and appear to be boiling. At least two lava flows from Mauna Loa filled earlier river channels. The oldest, the `Anuenue flow, is 10,500 years old. It forms the thick lip of Rainbow Falls and most of the rounded, gray boulders at Boiling Pots.

What the Hawaiians probably realized (and what you see today) is that many of these boulders are frozen into a younger pahoehoe flow, named after the Punahoa ahupua`a and about 3,100 years old. Tracing the flows up and down the Wailuku tells a geologic story of a river that had already deeply cut into Mauna Kea and Mauna Loa flows when it was filled by the `Anuenue flow over 10,000 years ago, probably creating clouds of steam. The filling caused the river to shift in many places and resume its erosional downcutting before lava again ventured into the river 3,100 years ago. These events took place before anyone could have witnessed them, yet the Hawaiian and the geologic versions of the Wailuku River story have many similarities. Giant reptiles never lived in Hawai`i, but nonetheless be respectful of pools of water within the river—a favorite resting place of the mo`o.

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a network of tubes toward the coast near the eastern boundary of Hawai`i Volcanoes National Park. The tube system is well-established, and surface flows from breakouts of the tube system are seen only occasionally in the coastal flats these days. Lava is entering the ocean mainly at three locations: Waha`ula, Kamokuna, and at a site 500 meters (550 yards) to the east of Waha`ula. The public is reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The active lava flows are hot and have places with very thin crust. The steam clouds are highly acidic and laced with glass particles.

One earthquake was reported felt during the past week ending on June 22, but our seismic network did not record an event at the reported time.