Continuous monitoring by the Pacific Islands Ocean Observing System (PacIOOS) provided a unique opportunity to study the effects of two tsunamis on the coastal and estuarine waters of Hawai’i. By the time the 2010 Chile and 2011 Japan tsunamis reached the waters of southern O’ahu, they had lost much of their power (both were < 1 m high in Honolulu Harbor). Nevertheless, their effects on the surrounding waters were profound, with increases observed in near-bed current velocities, mixing in estuarine waters, salinity, turbidity, chlorophyll, and pathogenic bacteria such as Vibrio spp. In addition to these increases, we observed small decreases in nitrate and dissolved oxygen concentrations offshore, including a dampening of the normal diel cycle in dissolved oxygen. Some of the effects penetrated canals as far as 1 km inshore and could be observed up to 0.5 km offshore. Data from the PacIOOS sensors and our sampling show that altered and potentially degraded water quality can persist longer than the physical threat from surge. Shortly after both tsunamis, the “all clear” signal was given and people resumed recreational activities in coastal waters before our data indicated recovery of healthy water quality conditions. Following such events, monitoring should be expanded and continued in order to accurately characterize water quality and evaluate potential public health risks.

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