A New Approach to Quantifying Riparian Buffers in the Rural-Urban Fringe in the Lower Fraser Valley

The importance of riparian buffers to protect water quality and fish habitat in agricultural areas is well documented. However, when setting protection and restoration priorities it is not easy for community groups and governments to assess which buffers are the most effective and why. The objective of this study was to develop a set of easily measured indicators that could be used to set priorities. Land-based indicators that other studies have shown to have an effect on riparian buffer functions and can be quickly measured in a Geographic Information System (GIS) were selected. Indicators were measured at two scales: the reach and riparian corridor. These indicators were compared with water quality and in-stream physical habitat to assess their ability to predict in-stream conditions in three watersheds in the Lower Fraser Valley, British Columbia. Six key indicators were identified that predict the effectiveness of the riparian buffer. Significant relationships were found between percent channelization, percent forest and percent agriculture and dissolved oxygen; the number of barns per kilometer and ammonia; forest cover conditions and stream temperature; and percent of forest cover and the number of pieces of large woody debris and habitat complexity. Land indicator conditions of the reach were found to be better predictors of in-stream physical habitat, whereas land indicator conditions of the riparian corridor were found to be better predictors of water quality. An easy-to-use assessment procedure for stakeholders to set restoration and protection priorities for riparian buffers was developed based on the observed relationships and thresholds. Introduction A riparian buffer is a vegetated zone occurring between a waterway and an adjacent area that is impacted by adjacent land uses (Castelle et al. 1994). It may extend past the natural riparian area. Through numerous research efforts many functions of riparian buffers have been documented from minimizing temperature fluctuations, to providing structural stream elements, to capturing nutrient runoff (Barton et al. 1985; Castelle et al. 1994; Hachmoller et al. 1991; Fennessey and Cronk 1997; Millar et al. 1997; Osborne & Kovacic 1993; Snyder et al. 1998; Wegner 1999). A variety of land uses and impacts may alter the effectiveness of the riparian buffer, such as drainage patterns, impervious surfaces, vegetation types and agricultural practices. The encroachment of agricultural land uses into the riparian buffer decrease its viability. Forest cover and wetlands have decreased significantly since European settlement in the 19th century with the increase of agricultural and urban areas (Boyle et al. 1997). In agricultural areas activities such as maximizing arable land, channelization of the stream, drainage tiles piped directly to the stream and vegetation removal reduce the effectiveness of the riparian buffer to enhance habitat diversity and filter nutrients and sediment (Bolton and Shellberg 2002; Stauffer et al. 2000). The intensification of agriculture puts further pressure on existing riparian buffers. In the Lower Fraser Valley agricultural intensification is evident in census data, for example the number of chickens per hectare by all farms in the Lower Fraser Valley changed by +39% from 1986 to 1996 (Schreier et al., 2003). Protection of existing buffers is an important method to minimize the impacts of agricultural intensification and urban encroachment. Riparian and aquatic ecosystems are currently being impacted at a greater rate than ever before (Kauffman, 1997). In the U.S. 70-90 % of riparian areas have been significantly altered. Where buffers have been impacted stream restoration is commonly employed as a method to attempt to re-establish the structure and function of a stream ecosystem (Zandbergen 2000; Nehlsen 1997). Restoration is labour intensive and expensive, therefore it is important to select areas that have the greatest potential for success. A riparian buffer assessment provides a method for highlighting areas where riparian buffers are currently functioning well and areas where stream restoration could enhance the buffer. Few riparian buffer assessments have been developed, particularly in agricultural areas in the Pacific Northwest. A similar assessment, the Index of Riparian Integrity (IRI), focused on urban impacts was developed in Western Washington (May et al. 1997). A riparian buffer assessment typically employs a set of indicators that can be used to predict and compare the integrity of a stream. Indicators allow us to efficiently measure a few key parameters, as opposed to exhaustively sampling every conceivable, measurable parameter 2003 Georgia Basin/Puget Sound Research Conference 2 • PROCEEDINGS Elliot: A New Approach to Quantifying Riparian Buffers in the Rural-Urban Fringe. PROCEEDINGS • 3 for use in investigating broad-scale issues (Cairns et al. 1993; Innis et al. 2000). A number of indicators are usually used to improve predictability (Kelly and Harwell 1990; Karr 1993). Determining the scale to measure indicators often proves difficult. Studies have shown that the influence of environmental factors on stream quality may act at a number of scales including watershed, riparian corridor and reach (Lammert and Allan 1999; Richards et al. 1996; Allan et al. 1997). Frissel et al. (1986) speculates that habitat and shade characteristics are affected by the local, reach scale, whereas temperature moderation, nutrient retention and channel form are affected by the regional or catchment scale. Successful restoration projects require attention to local and watershedscale goals (Kauffmann et al. 1997). In order to effectively develop indicators to predict the effectiveness of a riparian buffer at protecting both water quality and physical fish habitat parameters a variety of scales must be employed. The objectives of this study were to select a suite of land-based indicators that could potentially represent the effectiveness of a riparian buffer to maintain water quality and fish habitat; to investigate relationships between the land based indicators and water quality and in-stream physical habitat parameters; to determine the appropriate scale to measure the indicators at; to develop significant relationships and evidence of thresholds into an easy to apply riparian buffer effectiveness assessment. The study was carried out in three predominantly agricultural watersheds in the Lower Fraser Valley, British Columbia. Methods Study sites Sites were established in three watersheds in the Lower Fraser Valley, British Columbia: Elk Creek, Miami River and Salmon River Watersheds. These watersheds are predominantly agricultural with low density residential areas. The Elk Creek watershed has steep, forested headwaters that flow into a lower gradient, open, agricultural area in the lower watershed. The Miami River Watershed begins amongst flat, open agricultural land uses, flows through a forested area and out alongside a small town, Harrison Hotsprings. The Salmon River Watershed begins in a low gradient agricultural area. Downstream agricultural and residential areas predominate with an extensive forested buffer. The lower Salmon Watershed is dominated by flat, open agriculture. The agriculture in the three watersheds is mainly dairy and chicken barns, pasture, corn, berry and hobby farms. Indicator selection and sampling Indicators were selected that could be easily sampled using orthophotos and GIS. Other studies that have demonstrated which indicators may affect the following four stream functions: stabilizing stream banks, capturing nutrients and sediment, providing structural diversity in the stream, and moderating stream temperature were used to further focus the initial indicator selection. The following indicators were selected from an original larger set: percent forest; percent shrub; percent agriculture; percent pasture; percent row crop; percent channelization; and the number of barns (see Elliott, 2003 for more information). Indicators of urban impacts, percent impervious areas and number of breaks, were initially included, however due to the lack of in-stream parameters, peak flows and benthic invertebrates sampled that could reflect these urban impacts were not studied further. These indicators were sampled at the reach and riparian corridor scale. The riparian corridor was selected over the watershed scale because other studies have found it to be equally if not more predictive than the watershed, and it is simpler to measure. Johnson et al. (1997) and Addah (2002) found the riparian corridor to be an equal or better predictor of water quality than the watershed. At both the reach and riparian corridor scales the indicators were sampled in a 30m wide buffer on either side of the stream. At the reach scale indicators were sampled 200m upstream from the water quality site. At the riparian corridor scale indicators were sampled the entire distance upstream of the station to the headwaters. The number of barns were counted within the two scales. At the riparian corridor scale the barns were recorded as per km. Percent channelization is the length of stream that has been straightened divided by the total length. The percentage of land cover and land use variables were simply calculated by dividing the area that these indicators occupied by the total area at the scale of sampling. In-stream sampling In-stream sampling was used to determine the validity of the selected indicators. Water quality was sampled in September 2001, November 2001 and February 2002 at 35 sites. November sampling was used in the analysis presented, with the exception of stream temperature, which was analyzed using the September data. The water quality parameters that were sampled and used within the analysis were: dissolved oxygen, temperature, ammonia and nitrate. Stream temperature was also recorded hourly at 12 of the 35 sites using temperatur