By Chris Schnepf
Precipitation has a large influence on forests and how they function. Forests, and how they are managed or disturbed, also have huge effects on streams that flow from them and all related stream benefits, from fisheries to irrigated agriculture.
Because high water quality (low temperatures, low amounts of sediment) is such an important value of forest streams, extra care is taken in forest management to maintain that quality. People often presume that timber harvesting in and of itself is the greatest threat to water quality, mentally envisioning sediments eroding from harvested slopes. But generally, you do not get much erosion from the soil surface of a harvested area, unless the soils are intrinsically prone to slumping (those soils may slump regardless of harvesting). Even after a harvest, the tree root systems remain, and soils are further bound to the site by understory vegetation and high levels of organic matter.
The biggest threat to water quality is less the harvested surface than it is the surface of roads and skid trails used to access that timber. Most sediment from those areas comes during peak stormflow events, such as a rain-on-snow event. Therefore, the primary focus to maintain forest water quality (in addition to buffer zones adjacent to streams with sufficient trees to provide shade and other stream benefits) is on forest infrastructure – the roads and skid trails used not only for timber harvest, but also for stand management efforts such as planting or thinning, firefighting, and for outdoor recreation.
So how do you minimize sediment from forest roads and skid trials? One part of the solution is to limit roads and skid trails to only those necessary to manage forests and fires. But other techniques involve making roads and skid trails more prepared for peak storm events. If a road or stream-crossing fails during these events, large amounts of sediment can be sent to streams.
For example, roads must be designed and built to quickly drain water off a road and back into the forest (e.g., by the layout of the road but also by road drainage devices such as drivable dips and water bars). Roads can be closed during wet seasons to reduce the chances of failure. Stream crossings must be built to handle the largest stream flows. State forest practice laws set mandatory best management practices (BMP’s) to make sure forest infrastructure is up to these tasks.
But much of American forestry, and by extension, forest practice rulemaking, is based on how forests have functioned in the past. The past may not be a sufficient predictor for the future when factoring in climate change. Many of the current infrastructure guidelines are based on historical weather events (e.g., “the 50-year flood”). But what if future events exceed what we have experienced in the past? Many climate scientists project more extreme storm events and more rain-on-snow events than we have had historically. Such events would be exacerbated by major fires projected using climate models and steadily increasing fuel loads on many forests. Extreme fire behavior leaves fewer trees in buffer zones and can cause more damage to soils. The combination of more extreme weather and fire can produce runoff and sediment loads that could overwhelm forest infrastructure.
Therefore, some groups in western North America are taking a second look at their guidelines for stream crossings, looking more closely at things like larger or open-bottom culverts (see publications below) or better yet, bridges. Some of these approaches may also help fish pass more easily through these structures, which allows their free movement to colder water refuges during periods of higher stream temperatures.
So, what does this mean for Pacific Northwest forest owners? Most of our current road drainage installed in the last few decades should hold up fairly well in the future if it was well designed and is regularly maintained (e.g., cleaning out road drainage culverts). But if you are doing a timber sale that involves a stream crossing, you might want to err on the side of a larger culvert or a bridge in some locations to prepare for future events. Larger devices cost more but consider the cost of rebuilding a road that has blown out in a large storm. The sedimentation from such a blow-out can also be considerable. At least if you upgrade a bit while you are logging you are already paying for most of the installation cost. If you are thinking about this, it also makes sense to interact with your local state forest practice forester at your site. Some streams and soils are more vulnerable to peak events than others, and they can provide some on-site guidance on this.
For more information on forest infrastructure, see:
Idaho Forestry Best Management Practices Field Guide: Using BMPs to Protect Water Quality. Online Access
Adapting Resource Road Infrastructure to Climate Change (Technical report no. 61) Online Access
Effects of Climate Change on Infrastructure (in USDA Forest Service RMRS-GTR-375): Online Access