Turbidity Changes During Culvert to Bridge Upgrades at Carmen Creek, Idaho.

摘要

Forest Service, BLM, and state forest roads provide access for timber harvest and recreational use. Culverts used on these roads were historically designed to convey water under the road with little attention given to passage of aquatic organisms. In the past decade or so, driven largely by the Endangered Species Act listing of various salmonids in the Pacific Northwest, managers have designed and installed culverts to accommodate passage of aquatic species. Many existing culverts designed under the old criteria are impediments to passage and are replaced with culverts or bridges designed to accommodate aquatic species passage. Managers weigh the short-term negative impacts of sediment in the stream during culvert replacement against the positive long-term impacts of improved passage when considering which culverts to replace. Regulatory agencies, such as National Marine Fisheries Service, are requiring improved quantification of the short- and long-term impacts. One frequently used metric for the short-term impacts is turbidity. Allowable turbidity standards for surface water quality are set individually by each state. Idaho State criteria for cold water aquatic habitat require that turbidity not exceed the background level by more than 50 NTU (nephelometric turbidity units) instantaneously or more than 25 NTU for more than 10 consecutive days (IDEQ 1994). Jakober (2002) studied a culvert replacement in the Bitterroot National Forest in Montana and reported that 95 percent of the construction-related sediment occurred in the first 2 hours after diversion removal. He stated that sediment concentrations decreased to near pre-project levels within 24 hours.