What are Cryptorheic basin Wetlands?

Cryptorheic vs. Endorheic: Several of the wetlands in South Maui are in the form of cryptorheic basins.  A  cryptogenic basin has no visible outlets but is estimated to have underground outlets that reach the ocean eventually. They differ slightly from an Endorheic basin which never drains to anywhere. Endorheic is an area where a stream terminates with no known outlet to a body of water, for example, either a lake or the ocean.

Cryptorheic indicators: These cryptorheic basins are sometimes indicated on drainage maps where streams terminate mysteriously in the middle of neighborhoods on the coastal floodplain. It is theorized that some of these cryptorheic basins in Hawaii were formed during the Holocene period by the process of erosion and accretion of silt and eroded materials that form the floodplain shelf. The shoreline processes also built a dune system along the coastal edge that forms a seawall that is higher than the lower-lying parts of the basin in the back beach areas.

Cryptorheic wetlands: These cryptorheic basins were home to many perennial wetlands in former times. But due to changing climate, rainfall, deforestation, and altered water usage patterns, these once-perennial (permanent) wetlands have become intermittent (aka “ephemeral”).  Some of these land-locked wetlands were also altered with the addition of drainage channels to eliminate excess water. Two of these major cryptorheic basins were at the foot of the Keokea Gulch in the area known as Welakahao wetlands and the landlocked Halama wetlands. And the other, lesser-known, cryptorheic basin is at the foot of the Waimahaihai Gulch in the area that is now occupied by the fire station at the northern end of Kalama Park.  Interestingly here there is a heiau/temple that is thought to be associated with the fishing in this area. 

Why is this important? Unfortunately over development of this area has caused the once-extensive cryptorheic receiving basins of Kula Kai streams to be greatly reduced, as homes and businesses were built into the former wetlands. This creates a setup where urban flooding is likely, and this is borne out by our experiences living on the flood plain. Living in a cryptorheic basic is equivalent to living in a swimming pool or large bathtub that fills up every time when it rains.  Therefore urban planners need to pay closer attention to these geological features and to their effects on the watershed, drainage patterns, flooding potential, and their relationship to wetland environments.  

Cryptorheic-Wetland-Keokea-gulch-stream-Maui-Hawaii. Photo: S&DDorn

Further Reading:

Impact of Sea-Level Rise on the Hydrologic Landscape of the Mānā Plain, Kaua‘i, “These and other low-lying, cryptorheic regions formerly supported perennial wetlands [23], that expanded in wet and contracted in dry periods.”

Gomez, Basil. (2021). Impact of Sea-Level Rise on the Hydrologic Landscape of the Mānā Plain, Kaua‘i. Water. 13. 766. 10.3390/w13060766.