82 



lull. None of the other streams on (3ahu have discharges during 

 dry weather in excess of three million gallons daily at elevations 

 of five hundred feet or more above sea level. The Kahana 

 stream has probably the greatest low water flow, while the low 

 flows of the Waiahole and Punaluu streams are nearly as large. 

 The Waikane stream is much smaller, but as it lies between the 

 Waiahole and Kahana, its comparatively small flow is of con- 

 siderable importance in augmenting the flow of the three larger 

 streams. The flood discharges of all of these streams are very 

 high and of much more frequent occurrence than on the leeward 

 side of the island. 



The rainfall along the windward palis of these valleys is prob- 

 ably about twice that on the corresponding slopes on the leeward 

 side of the Koolau range. On account of topographic and 

 geologic conditions, however, the percentage of surface run-off 

 to rainfall on the windward side is much greater than on the 

 leeward side. It is estimated that at least fifty per cent, of the 

 rainfall on the leeward side becomes ground water which sup- 

 plies the various and separate artesian supplies of the leeward 

 valleys. It is probable that formerly a much larger proportion of 

 the rainfall on the windward side became ground water, a large 

 part of which appeared as springs in the lower valleys and near 

 sea level. Erosion apparently has eaten into the upper reaches 

 of these valleys until the old porous surface strata have largely 

 disappeared and the hard, water-impervious strata beneath have 

 been exposed. As a result, these windward valleys have now- 

 exposed many high level springs which bring to the surface un- 

 derground water courses that formerly came to the surface at 

 lower levels, or below sea level. The result has produced a much 

 higher low water flow from the upper reaches of these valleys 

 than at other localities on the island. 



The \\^aiahole \\^ater Company's project will divert the surface 

 flow from the Waiahole. Waianu, Waikane and Kahana valleys 

 at an elevation of from 800 to 753 feet above sea level. From 

 available data it is estimated that the average low water flow 

 diverted will be about thirty million gallons per day, and that 

 the average yearly flow will be between forty and fifty million 

 gallons per day. The frequent flood run-off will undoubtedly 

 amount to more than the capacity of the project, which is 125 

 million gallons per day. 



Underground JVater. 



The north, or windward, portal of the main tunnel was located 

 directly under the main springs which furnish the low flow of 

 the AA'aiahole stream. It was anticipated that the tunnel would 

 encounter the underground channels which fed these springs and 

 the main tunnel was started about three feet below grade and 

 was gradually worked up to the real tunnel grade, to allow the 



