The Functions of Riparian Buffers in Urban Watersheds

There are numerous functions that the riparian zone provides for the streams in undisturbed watersheds in the Pacific Northwest. In general the vegetation in this zone influences the water quality, hydrology, and biology of the streams. This paper will briefly look at how the riparian buffer interacts with the stream to moderate temperature, reduce sediment and nutrient loads, attenuate peak flows, and maintain the biological integrity of the stream. Once the functions provided by riparian areas in undisturbed watersheds have been evaluated, this paper will then focus on the ability of a riparian buffer to provide the same functions in urban watersheds. Urban watersheds have many unique characteristics, and the associated streams have altered water quality, hydrology and biology. To address this problem, government agencies enforce riparian buffer widths in an attempt to minimize the impacts of development on the stream. However, in urban watersheds, the buffer may not be an effective method to reduce the degradation of urban streams due to the increased volume of stormwater, which is often channelized through the buffer. The buffer is bypassed, and therefore it is not effective at reducing peak flows or the sediment and nutrients carried by the stormwater. An evaluation of the effectiveness of a riparian buffer in moderating stream temperatures was done using data collected in two watersheds, Rock and Richardson Creeks, in the Portland, Oregon, Metro area. Portland Metro, the regional government, is interested in the condition of these watersheds because they both lie within the urban growth boundary and therefore the amount of development in these watersheds is expected to double over the next 50 years. The establishment of riparian buffers is one methods of regulation that will be implemented in these watersheds to protect the streams from urbanization. In an attempt to evaluate the effectiveness of these methods, the function of temperature attenuation was examined in each of the watersheds. The percent of the riparian buffer that was intact upstream of each sampling site was correlated with the maximum, minimum, and daily fluctuation observed throughout the summer of 1996. The general trend observed was an increase of all three measures of temperature as the percent of intact buffer decreased, which supports the hypothesis that at least some of the conditions of the stream are related to the condition of the riparian buffer