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 the islandof O'ahu in the Hawaiian Islands have been hardened to prevent flooding. Thehardening process has involved elimination of the natural riparian habitat andreplacement of the natural stream channel with a concrete-lined conduit havingvertical walls and a broad, flat bottom. The shallow depth of the water columnand absence of shade have resulted in temperatures that average as much as4-5°C above ambient and rise as high as 32°C during daylight hours. Unlikemost low-order streams, the hardened sections of both streams are autotrophic,as evidenced by elevated pH values and O2 concentrations as high as 150% ofsaturation. Several allochthonous inputs, one from a storm sewer and the otherfrom a natural spring, introduced water with anomalously low O2 concentrationsand very high nitrate concentrations. The absence of sediments in thehardened sections of the streams precludes natural sedimentary microbial processes,including denitrification. Nitrate concentrations in a section of WaimanaloStream with a natural streambed drop dramatically from values in excess of400 11M to concentrations less than 10 11M at the head of the estuary. Althoughsome of this decline is due to dilution with seawater, the concentration of nitrateat the head of the estuary is only 10% of the value that could be explained bydilution effects. Biological processes associated with a natural streambed thusappear very important to functionality of the streams and in particular to theirability to process allochthonous nutrient inputs in a way that minimizes impactson the nearshore environment. Prevention of flooding can be accomplished bymechanisms that do not involve elimination of riparian buffer zones and destructionof channel habitat. To maintain water quality and stream functionality,it is important that these alternative methods of flood control be utilized. Convertingnatural streams to storm sewers is an unenlightened way to addressflooding problems.