A high degree of correlation was found between environmental variables. The proportion of fine inorganic sediment (<0.25 mm) in streambeds was shown to increase with increasing values of a principal component describing road age, road drainage surface area, and drainage vegetation cover. The logging of upstream catchments was shown to increase sediment input upstream of the crossings. The proportion of coarse particles deposited downstream was found to increase with increasing gradient of streamside drainage channels, while the proportion of deposited fines increased with increasing vegetation cover of the channels.

Overall animal densities were found to be higher downstream due to an increased abundance of oligochaetes. Numbers of Eustheniidae and Austroperlidae (Plecoptera) were lower at downstream sites. Increased proportions of fine inorganic sediment were correlated with lower numbers of in-stream taxa. These results support the hypotheses that in-stream invertebrate community composition is altered by sediment input from road crossings and that the characteristics of road crossing design are influential in controlling the input of roading-derived sediment to streams.

In addition to the parameters of crossing design, characteristics of the roading and other aspects of the catchment were measured at each location and analysed as potential treatment variables for the sedimentary and biotic responses.