Posted 7/02/2010
The U.S. Geological Survey recently published its report on the impacts that stream diversions and other structures to control stream flows have had on Na Wai `Eha, the four major streams flowing down the eastern slopes of the West Maui mountains.
The study, by Delwyn Oki, Reuben Wolff, and Jeff Perreault, is available here: Effects of Diversions on Na Wai `Eha
Our review of this important work follows:
Delwyn S. Oki, Reuben H. Wolff, and Jeff A. Perreault. Effects of Surface-Water Diversion on Streamflow, Recharge, Physical Habitat, and Temperature, Na Wai `Eha, Maui, Hawai`i. U.S. Geological Survey Scientific Investigations Report 2010-5011
Dying fish, gulping for water in shallow puddles that will soon disappear in the heat of the day. Concrete-lined channels that give no quarter for native Hawaiian freshwater fish that, like salmon, need to climb upstream to complete their life cycles. Stream beds where water is a distant memory in all but times of flood. Grates that span the width of a watercourse, making it all but impossible for some species of unique Hawaiian fish to reach the cool, high elevations where they would naturally spend their adult reproductive lives.
These are among the heart-rending images in a recent report by the U.S. Geological Survey on the effects that more than a dozen diversions have had on the four great waters – Na Wai `Eha – of Central Maui, the same streams whose flows were the subject of a four-year-long contested case hearing before the Hawai`i Commission on Water Resource Management. Given the commission’s decision in that case, released in June 2010, it is not likely the habitat for native aquatic life will be significantly increased any time soon.
The report was not published in time to be referenced in the Water Commission’s decision. Still, the commission was aware of the study (it was a cooperating agency, along with Maui County and the state Office of Hawaiian Affairs) and lead author Delwyn Oki testified during the contested case hearing, providing the commission with the study’s conclusions.
One of the chief contributions the study makes is estimating the volume of water needed in the mauka portion of streams to ensure their continuous flow to the ocean. For three of the four streams, the volume is almost trivial compared to median flows. For Waihe`e River, it’s 1 million gallons a day (mgd), compared to the median flow of 34 mgd above the highest diversion. For Waiehu, just 1.3 mgd total in the North and South branches is sufficient for a continuous flow to the sea 95 percent of the time (the total median flow for both branches of Waiehu is 6.4 mgd). `Iao Stream would need to see just a fifth (5 mgd) of its median flow of 25 mgd in order for water to be in the entire length of the stream. Waikapu Stream, the only one of the four to flow south, feeding into the Kealia Pond near Ma`alaea, is also the only stream among the four that probably would see dry periods along its reach, even without diversions. The USGS estimates that a minimum flow of 6.8 mgd would be needed for continuous flows – an amount that is greater than the estimated median flow of 4.3 mgd.
The impact of restored flows on habitat for native stream fauna would be tremendous. The authors write: “A generalized relation between physical habitat and discharge developed for this study indicates that if diversions reduce stream flow to half of the natural, undiverted Q70 discharge (discharge that is equaled or exceeded 70 percent of the time), which is an indicator of median base-flow conditions, then habitat will be reduced to about 80 percent of what it would be at the Q70 discharge.” In other words, habitat quality is maintained at a pretty high level even when flows are reduced – so long as that reduction still leaves water flowing in the stream.
But under current conditions, even without taking into account smaller auwai, the main diversions at times of low flows “reduce habitat for native stream fauna by: 100 percent for Waihe`e River near an altitude of 45 feet; more than 60 percent for Waiehu Stream near an altitude of 20 feet; 100 percent for `Iao Stream near an altitude of 35 feet; and more than 90 percent for Waikapu Stream near an altitude of 400 feet.”
Hourly photographs taken by cameras the USGS placed on streams in 2006-2008 showed that some streams below diversions were dry more than 50 percent of the time. “Many of these reaches would be perennial or nearly perennial in the absence of diversions,” the USGS found.
Under the Water Commission’s decision in the Na Wai `Eha contested case hearing, `Iao and Waikapu streams can continue to be completely dewatered by diversions, with no minimum flows remaining in the streams’ lower reaches. After diversions, Waiehu Stream is left with 2.5 mgd in its lower reaches. With an allowance for 1.9 mgd lost to the underlying aquifer at lower elevations, about .6 mgd will be left at the mouth of Waiehu to flow into the sea. Waihe`e River, which is estimated to lose about 4 mgd into the aquifer at lower elevations, will have about 6 mgd on average entering the sea.
The USGS report focuses on another less well known aspect of stream diversions: the effects they have on aquifer recharge at lower elevations. At higher elevations, Na Wai `Eha tend to be fed not just by rain and runoff, but also by underground water stored behind impervious rock formations. At lower elevations, where the streams cut through valley floors, they feed basal aquifers near the shore.
“Existing surface-water diversions in Waihe`e River, `Iao Stream, and Waikapu Stream are capable of reducing the amount of recharge during periods of low flow by more than 80 percent relative to undiverted conditions, and existing diversions in Waiehu Stream are capable of reducing the amount of recharge by more than 33 percent,” the study notes.
The study’s authors suggested that stream flows be restored “at different rates for extended periods (exceeding a few weeks)” to allow them a chance to better characterize the effects of diversion on recharge and physical habitat. The commission rejected the idea.
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