The use of sediment budget concepts to assess the impact on watersheds of forestry operations i.....Introduction and objectivesSediment budget studies: principles and previous workThe study area, study design, and history of the projectMethodologyStream dischargeTurbidity and suspended sediment concentrationSuspended sediment yieldBedloadSolutesSediment source inventoriesResults and discussionRunoff, turbidity, and hysteresisSuspended sediment yieldSediment sourcesBedload yieldDissolved solidsLong-term average sediment and solute yieldsFactors affecting sediment yield and sensitivity to developmentConclusionsAcknowledgementsReferencesThe use of sediment budget concepts to assess the impacton watersheds of forestry operations in the southerninterior of British ColumbiaPeter Jordan⁎British Columbia Ministry of Forests, Kootenay Lake Forestry Centre, 1907 Ridgewood Rd., Nelson, BC, Canada V1L 6K1Accepted 29 September 2005Available online 20 March 2006AbstractSediment budget concepts can be applied to the assessment of the impacts of forest resource development on the sedimentregime of streams. In British Columbia, these impacts are of concern because of the extent of commercially valuable forests.Increases in sediment yield from forestry operations can affect water quality, fish habitat, and channel stability. To address theseconcerns, the BC Ministry of Forests conducted several sediment budget studies from about 1992 to 2002. This paper reports theresults of two studies, focusing on the water quality of streams used for community water supply. The studies address severalquestions: How sensitive are the streams to an increase in sediment supply? Are development-related sediment sources significantcompared to natural sources? What forest practices are responsible for increasing or minimizing sediment impacts? How can theimpact of forestry operations on water quality be monitored? The studies used a paired watershed approach, with discharge,turbidity, sediment yield, and solute yield measured on watersheds undergoing logging and road building, and on similarundeveloped watersheds. The studies concluded that erosion from forest roads can be a significant source of suspended sediment,but sediment from logging operations is usually negligible. The risk of landslides is an important factor in the sediment budgets,but is difficult to quantify. Differences in geology and groundwater regime can influence the sensitivity of watersheds to sedimentimpacts.© 2006 Elsevier B.V. All rights reserved.Keywords: Sediment budget; Turbidity; Erosion; Forest roads; Forest practices; British Columbia1. Introduction and objectivesThe water quality of streams used for water supply inthe southern interior of British Columbia is an importantissue. Many water users in rural areas depend on surfacewater, and use it with little treatment. Most cities andtowns in the region obtain their water from mountainstreams, although they employ reservoirs and treatmentsystems. The watersheds supplying these streams arepart of the provincial forest, and are used for timberharvesting. Water quantity and quality, the sedimentyield of streams, and the effects of timber harvesting andforest roads on these, are important concerns for forestmanagement. Fish habitat and flooding are additionalconcerns on some streams.The British Columbia Ministry of Forests has con-ducted research programmes on sediment budgets,erosion and sedim entation issues related to forest de-velopment, and on streamflow and snow hydrology,Geomorphology 79 (2006) 27 – 44www.elsevier.com/locate/geomorph⁎Tel.: +1 250 825 1119; fax: +1 250 825 9657.E-mail address: [email protected]/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.geomorph.2005.09.019beginning in the early 1990s. This paper will discusssome results of this research in the Kootenay region ofsoutheastern British Columbia, a region where commu-nities are particularly dependent on surface water, andwhere forest development in watersheds is often a con-tentious issue.The objectives of this paper are:1. to summarize the past 10 years of watershed researchin this region, especially with respect to sedimentbudget results;2. to compare sediment and solute budgets in water-sheds with different bedrock geology and soil tex-ture, and in different hydrologic environments;3. to illustrate the practical application of sedimentbudget concepts to assessing the impacts of forestresource development on streams and water quality.Some additional objec tives of the research pro-gramme, which have been discussed in more detail else-where (Jordan and Commandeur, 1998; Jordan, 2001),include: to identify natural and development-relatedsources of sediment; to investigate the effect of roadengineering an d forest harvesting practices on sedimentproduction; and to collect data on hydrologic eventswhich are significant in sediment production.2. Sediment budget studies: principles and previousworkA sediment budget study requires quantificationof the sources of sediment, changes in sediment stor-age in the stre am channel network, and sediment dis-charge at the stream outlet. This can be expressed as anequation:I ¼ O þ DSwhere I is input, O is output, and S is change in storage.The equation can be applied at any scale of interest,from an entire watershed to a single reach of streamchannel. In practice, the equation is often applied sep-arately to suspended sediment (fine sand, silt, and clay),and bedload (coarse sand and gravel), since the physicsof transport and storage of the two types of sedimentare different. The theory and methodology of sedimentbudget studies have been described in the literature(Swanson et al., 1982; Reid and Dunne, 1996). The solutebudget of a watershed can be expressed in the sameterms, although once dissolved solids reach the streamchannel system, storage (except in arid environments)is negligible.A sediment budget framework for the study of geo-morphic processes in forested drainage basins was de-veloped in the Pacific Northwest region of the UnitedStates in the 1970s, where it was applied to investigatingthe response of stream channels to extreme meteoro-logical events and to disturbance from forest develop-ment (Dietrich and Dunne, 1978; Swanson et al., 1982).The importance of sediment from forest road erosionwas realized at this time, and several studies addressedthe role of sediment from forest roads on the sedimentbudget (Beschta, 1978;