The potential for road-associated spills and chemicals applied to roads to enter streams. The study also noted that where road drainage points were a sufficient distance from streams, the drainage discharge re-infiltrated and did not contribute sediment to streams. Piehl et al. Outlet erosion increased with increased ditch length. They noted that the most effective method of preventing road sediment from reaching streams is to drain ditches onto the forest floor where it can infiltrate before reaching a stream.
Roads frequently generate overland flow from relatively impermeable running surfaces and cutslopes.
In addition, the interception of interflow at cutslopes can generate substantial amounts of runoff, converting subsurface flows to surface flows. Where these surface flows are continuous between roads and streams, such as where inboard ditches convey road runoff to stream channels, the road generating or receiving the runoff is considered hydrologically connected to the stream network. In other words, a hydrologically-connected road becomes part of the stream network. Wherever a hydrologic connection exists, accelerated runoff, sediments and road-associated chemicals, such as spills or oils, generated on the road surface and cutslope have a direct route to the natural channel network and surface waters. Hydrologic connectivity will tend to increase with increasing intensity and duration of precipitation or snowmelt, and with increasing antecedent soil moisture content.