Impact of Stream Hardening on Water Quality and Metabolic Characteristics of Waimanalo and Kane'ohe Streams, O'ahu, Hawaiian Islands

摘要

Kane'ohe and Waimanalo Streams on the windward side of die island of O'ahu in the Hawaiian Islands have been hardened to prevent flooding. The hardening process has involved elimination of die natural riparian habitat and replacement of the natural stream channel with a concrete-lined conduit having vertical walls and a broad, flat bottom. The shallow depth of the water column and absence of shade have resulted in temperatures that average as much as 4-5 ℃ above ambient and rise as high as 32 ℃ during daylight hours. Unlike most low-order streams, the hardened sections of both streams are autotrophic, as evidenced by elevated pH values and O_2 concentrations as high as 150% of saturation. Several allochthonous inputs, one from a storm sewer and the other from a natural spring, introduced water with anomalously low O_2 concentrations and very high nitrate concentrations. The absence of sediments in the hardened sections of die streams precludes natural sedimentary microbial processes, including denitrification. Nitrate concentrations in a section of Waimanalo Stream with a natural streambed drop dramatically from values in excess of 400 μM to concentrations less than 10 μM at the head of die estuary. Although some of this decline is due to dilution with seawater, the concentration of nitrate at die head of the estuary is only 10% of the value that could be explained by dilution effects. Biological processes associated with a natural streambed thus appear very important to functionality of the streams and in particular to their ability to process allochthonous nutrient inputs in a way that minimizes impacts on the nearshore environment. Prevention of flooding can be accomplished by mechanisms that do not involve elimination of riparian buffer zones and destruction of channel habitat. To maintain water quality and stream functionality, it is important that these alternative methods of flood control be utilized. Converting natural streams to storm sewers is an unenlightened way to address flooding problems.