Most of Kohala's summit groundwater ends up in either Waipi'o or Honokane Nui; the enormous amount of water routed through these valleys results in a large amount of water erosion, which causes the valley walls to frequently collapse, accelerating valley development. This abundance of surface water inspired the first of Kohala's three massive irrigation diversion ditches, the Upper Hamakua Ditch, although the water was initially used for transporting (fluming) sugarcane to the mills rather than irrigation. Surface water supplies, however, were not stable enough for the large scale plantation economy developing in Hamakua and North Kohala, so later ditches tapped the spring fed base flows of the valleys and larger gulches. Only Waimanu Valley escaped with none of its tributaries tapped, along with the 16 smaller streams between Waikoloa Stream and the Wailoa river in Waipi'o Valley.