Surface Water Protection Branch

Background

Hydromodification is the alteration of the flow of water through stream diversion, channelization, dams, levees, and other methods. While hydromodifications serve important purposes such as natural hazard protection, resource allocation, and improving land use options, they can also contribute to nonpoint source pollution.

Because of Hawaii’s sub-tropical climate, “flashy” storm events consisting of high peak stream flows and large volumes of runoff are common. County drainage standards were first developed in the 1960s to safely handle these runoff volumes, in order to protect life and property located close to streams. Consequently, the concrete channelization of urban streams changed their physical, chemical, hydrological, and ecological characteristics.

A frequent result of channelization and channel modification activities is the degradation or even elimination of instream and riparian habitats. From a pollution perspective, the loss of riparian ecosystems increases the transport of sediments and other nonpoint source pollutants through watersheds into coastal waters.