HYDROLOGICAL PROCESSES Hydrol. Process. 21, 3107–3122 (2007) Published online 8 May 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/hyp.6531 Catchment-scale contribution of forest roads to stream exports of sediment, phosphorus and nitrogen Gary J. Sheridan* and Philip J. Noske School of Forest and Ecosystem Science, The University of Melbourne, PO Box 137, Heidelberg 3084, Victoria, Australia Abstract: The relative contribution of forest roads to total catchment exports of suspended sediment, phosphorus, and nitrogen was estimated for a 13 451 ha forested catchment in southeastern Australia. Instrumentation was installed for 1 year to quantify total in-stream exports of suspended sediment, phosphorus, and nitrogen. In addition, a total of 101 road–stream crossings were mapped and characterized in detail within the catchment to identify the properties of the road section where the road network and the stream network intersect. Sediment and nutrient generation rates from different forest road types within the catchment were quantified using permanent instrumentation and rainfall simulation. Sediment and nutrient generation rates, mapped stream crossing information, traffic data and annual rainfall data were used to estimate annual loads of sediment, phosphorus, and nitrogen from each stream crossing in the catchment. The annual sum of these loads was compared with the measured total catchment exports to estimate the proportional contribution of loads from roads within the catchment. The results indicated that 3Ð15 ha of near-stream unsealed road surface with an average slope of 8Ð4% delivered an estimated 50 t of the 1142 t of total suspended sediment exported from the catchment, or about 4Ð4% of the total sediment load from the forest. Stream discharge over this period was 69 573 Ml. The unsealed road network delivered an estimated maximum of 22 kg of the 1244 kg of total phosphorus from the catchment, or less than 1Ð8% of the total load from the forest. The average sediment and phosphorous load per crossing was estimated at 0Ð5 t (standard deviation 1Ð0 t) and 0Ð22 kg (standard deviation 0Ð30 kg) respectively. The lower proportional contribution of total phosphorus resulted from a low ratio of total phosphorus to total suspended sediment for the road-derived sediment. The unsealed road network delivered approximately 33 kg of the 20 163 kg of total nitrogen, about 0Ð16% of the total load of nitrogen from the forest. The data indicate that, in this catchment, improvement of stream crossings would yield only small benefits in terms of net catchment exports of total suspended sediment and total phosphorus, and no benefit in terms of total nitrogen. These results are for a catchment with minimal road-related mass movement, and extrapolation of these findings to the broader forested estate requires further research. Copyright  2007 John Wiley & Sons, Ltd. KEY WORDS water quality; soil erosion; tracks; timber harvesting; unsealed roads; erodibility; Australia Received 9 June 2005; Accepted 23 June 2006 INTRODUCTION Sediment and nutrients are common pollutants of streams and water impoundments, and one of the challenges for catchment managers is to identify the dominant point and non-point sources of these pollutants within a catchment. This allows the limited resources for remediation to be carefully targeted to maximize the cost/benefit to the community. Increasingly, unsealed roads are being identified as key sources of sediment. Gruszowski et al. (2003) suggest that 30% of the sediment load from a rural UK catchment was generated from, or carried via, unsealed roads. In forests, unsealed roads have long been identified as a significant source of sediment that enters streams (e.g. Hoover, 1952; Weitzman and Trimble, 1955; Swift, 1984; Bilby, 1985; Burroughs and King, 1989; Fahey and Coker, 1989; Constantini et al., 1999; Luce and Black, 1999; Ziegler et al., 2000a,b, 2001, 2002; Cornish, 2001; Lane and Sheridan, 2002; Sheridan et al., 2006; Sheridan and Noske, 2007). * Correspondence to: Gary J. Sheridan, School of Forest and Ecosystem Science, The University of Melbourne, PO Box 137, Heidelberg 3084, Victoria, Australia. E-mail: sheridan@unimelb.edu.au Several studies have indicated that the levels of sedi- ment attributable to forest road networks are greater than the levels associated with timber harvest areas. Motha et al. (2003) used sediment-tracing techniques and esti- mated that between 18 and 39% of the sediment load from a Victorian forest was from unsealed roads, whereas harvest areas contributed only 5–15%. Levels of fine sediment that infiltrated into streambeds in Tasmanian forest streams were higher downstream than upstream from a road crossing, and substantially higher than levels recorded in upper tributaries within logged catchments (Davies and Nelson, 1993). In a 10-year study conducted by Grayson et al. (1993), monitored individual road seg- ments (4 m ð 100 m) produced as much sediment as a 30 ha logged catchment. Similarly, a 15-year logging and water quality study in New South Wales reported that the only observed increases in stream turbidity lev- els were attributable to road–stream crossings (Cornish, 2001). The area and slope of the road (Sheridan et al., 2003), erodibility of the surface material (Grayson et al., 1993), and the level and type of traffic (Reid and Dunne, 1984; Bilby et al., 1989; Sheridan and Noske, 2007) are Copyright  2007 John Wiley & Sons, Ltd. AUTHORS COPY