The effects of unpaved roads on suspended sediment concentration of third- to fifth-order streams- A case study from southern Brazil

Unpaved roads have earned a reputation of inducing adverse effects on downstream water resources by increasing suspended sediment concentration because they typically generate sediment at rates up to several orders of magnitude above background and because they may enhance the efficiency of sediment delivery to fluvial networks. Although much research has been conducted on road effects in forested landscapes, proper understanding of their hydro-geomorphic role in rural areas is still desired. Unpaved roads are fundamental in the agricultural systems employed for the cultivation of maize and black beans on topographically-steep, marginal lands of southern Brazil. Marginal lands generate a sizeable fraction of the agricultural production in the state of Paraná, one of Brazil's agricultural powerhouses. This study documents the localized impacts on suspended sediment concentration of seven unpaved road crossings in the Guarabiroba River Catchment, Paraná, Brazil. A total of 156 suspended sediment samples were manually collected both upstream and downstream of road-crossings between 22-Apr-09 and 26-Apr-10 during 14 rainfall events ranging between 16 and 96 mm in total rainfall. The average length of road directly delivering runoff to each crossing varied from 0.56 - 2.4 km, and the size of the catchment areas of the third to fifth order monitored streams ranged from 0.3 to 13.5 km2. In addition to stream samples, 78 samples representing unpaved road runoff were collected during the same rain events. Upstream and downstream mean concentration values were compared for each storm at every site based on a paired t-test analysis (0.05% level). Mean suspended sediment concentration at stream segments located upstream of road crossings was 0.04 mg L-1 (s.d. = 0.05 mg L-1), while the mean downstream concentration was 0.11 mg L-1 (s.d. = 0.14 mg L-1) or 2.9 times higher than upstream samples. Meanwhile, road runoff had an average concentration of 0.93 mg L-1 (s.d. = 0.97 mg L-1) or a value that was 26 and 9 times higher than upstream and downstream samples, respectively. Event-by-event comparisons for each individual site display a tendency for road crossings to statistically increase mean concentration by an average of 6.9 times relative to upstream values only for those streams with the smallest catchment areas (< 2.6 km2). Meanwhile, for stream segments draining an excess of 9.5 km2 there was a tendency for upstream and downstream mean concentration to be statistically similar, presumably due to the minimal amount of road runoff contributed at a single crossing relative to the discharge being carried by the streams. The localized increase in sediment concentration by unpaved road crossing was not found to be associated to road surface area nor average slope. In summary, these findings suggest that the effect of unpaved roads on the local concentration of suspended sediment being transported by streams is strongly scale-dependent in that the impact tends to be important for the low-order headwater streams and undetectable for their higher order counterparts. These results point to the importance of low-order stream crossings in degrading water quality and the need to further explore the role of unpaved roads as agents of degradation in rural areas.