U.S. "water resources" include streams, rivers, wetlands, estuaries, lakes, and coasts. Those resources support billions of dollars in commerce, provide drinking water, supply habitat for fish and wildlife, and provide recreational opportunities. The U.S. Army Corps of Engineers (the Corps) is one of several federal agencies that undertake water resource projects. The majority of Corps civil works projects involve commercial navigation, flood risk management, and ecosystem restoration. Congress generally authorizes Corps activities and provides policy direction in Water Resources Development Acts (WRDAs). Many studies of civil works projects prepared by the Corps are prepared in response to a project-specific authorization from Congress to study a water resources problem. Generally, the outcome of the Corps' study process is a Corps Chief of Engineers report to Congress (a Chief's Report) that supports the recommendation for Congress to authorize project construction. Project development involves multiple stages, from study initiation to planning and design and ultimately to construction and operation. Years or even decades may pass from the time the Corps is authorized to study a water resources-related problem and the Corps constructs a project to address that problem. In recent decades, few projects authorized for study have led to constructed projects. Of the projects that do proceed, various factors affect the time it takes to move from project planning to the completion of construction. Generally, the most significant factors in some way relate to the availability of federal funding and the time it takes to obtain necessary congressional authorizations. Recently, to expedite project development, congressional attention has focused on activities that must be completed before a Chief's Report can be submitted to Congress recommending project construction. These activities generally comprise the planning stage of project development. Pursuant to various statutory requirements, during project planning, the Corps is obligated to complete a potentially complex array of studies, reports, and evaluations. In an effort to expedite that project delivery, some groups have focused particular attention on the time it takes the Corps to comply with federal environmental requirements that must be addressed during planning. Given the congressional interest in this topic, this report assesses certain elements of the project planning process in an effort to identify whether or the degree to which meeting environmental requirements is a significant cause of delay in project delivery. To do so, the report provides background information on the potentially wide array of environmental requirements that may apply to a given civil works project (including why Congress established such requirements), how the Corps integrates compliance with those requirements into the project planning process, and how required elements of the planning process may affect project delivery. To provide necessary background, this report identifies selected issues that have arisen in the past 50 years that resulted in Congress enacting numerous environmental requirements that (1) directly affect the Corps' project planning process and (2) are intended to minimize adverse project-specific impacts. It also provides an overview of the Corps' environmental evaluation and compliance requirements, including how those requirements are integrated into the project planning process. More specifically, the report provides an overview of the federal requirements that obligate the Corps to evaluate the impacts of a given civil works project and the compliance requirements that generally must be addressed before the Corps will submit a Chief's Report to Congress. The Corps project development process is complex. By focusing on environmental requirements that must be addressed during planning, this report does not identify every element of project development that may affect project delivery. More specifically, it does not provide detail regarding other factors that have been identified as particularly relevant to the timing of project delivery—federal funding availability and the timing of necessary congressional approvals. As they affect the project planning process, selected issues associated with congressional funding and approvals may be discussed, but are generally beyond the scope of this report. For information about those issues, see CRS Report R41243, Army Corps of Engineers Water Resource Projects: Authorization and Appropriations , and CRS Report R41961, Army Corps Fiscal Challenges: Frequently Asked Questions , by [author name scrubbed] and [author name scrubbed]. The majority of civil works projects undertaken by the Corps fall within its navigation, flood risk management, and ecosystem restoration and environmental protection missions. Historically, navigation and flood risk projects involved a potentially wide range of activities such as the construction of levees, floodgates, or dams; channel deepening (i.e., dredging) or widening; or changing floodplain uses. Some of those projects resulted in adverse impacts such as unanticipated flood damage after alterations to wetlands or floodplains, the destruction of aquatic plant or animal habitat, or impaired or altered water quality after dredging or channelizing waterways or building locks and dams. The Corps' civil works mission was expanded to include ecosystem restoration projects, in part, to authorize the agency to address adverse effects from past civil works projects. Addressing those impacts involves actions such as restoring natural channel conditions, modifying obstructions to fish passage, or removing levees to restore wetland hydrology. Many of the environmental requirements that apply to Corps civil works projects today were put in place at the direction of Congress to ensure that project planning reduces the potential for unanticipated adverse impacts and/or identifies measures to minimize or mitigate unavoidable impacts. In reviewing the range of federal environmental requirements potentially applicable to civil works projects, the requirements may fall into the one of the following categories: Requirements to address certain environmental aspects of a proposed project during planning . These include requirements, established by Congress, that apply explicitly to federal investment in water resources development. For example, the Corps is obligated to conduct specific studies, reports, and evaluations to ensure that the environmental impacts of a project are identified and considered; evaluate a project's economic and public safety benefits compared to its adverse environmental impacts; and identify opportunities to protect, preserve, and/or enhance the quality of the environment. Requirements intended to protect human health or minimize harm to a protected resource . These include requirements that arise from federal laws, regulations, executive orders, or Corps policy that may apply as a result of project-specific impacts. Compliance requirements will be identified during planning and will generally depend on the degree to which project impacts adversely affect air or water quality, or natural or cultural resources specifically protected by Congress (e.g., historic sites, Native American graves, endangered species or their habitat). They may also include any requirements that direct the Corps to enhance the quality of the environment. Both sets of requirements are largely implemented by the Corps, but may require the Corps to consult with or obtain some level of review by an outside federal agency or a state or tribal agency authorized to implement a specific federal law (see Table 1 in the section " Impact-Specific Environmental Requirements "). As required by Congress, the Corps cannot recommend a project to Congress for construction authorization until it has evaluated and can describe "economic, environmental, and social benefits and detriments" of a recommended plan and possible alternative plans. A key element of its evaluation of environmental detriments and benefits involves identifying a project's impacts and environmental compliance requirements associated with those impacts. When there is debate over the degree to which environmental requirements affect project delivery, that debate often centers around the time it takes the Corps to demonstrate compliance with specific requirements. Most recently, that debate has centered around the time that it takes the Corps to complete some level of coordination with or obtain necessary approvals from other federal agencies (e.g., the Department of the Interior's U.S. Fish and Wildlife Service). The process required to obtain necessary federal reviews of civil works projects may be highly complex, among other reasons, in order to ensure compliance with protections established by Congress. As a result, they may require a high degree of resources agency involvement (e.g., in developing ecosystem restoration projects). To understand why a potentially complex array of environmental requirements may apply to water resource projects, it is useful to understand the social and environmental concerns that led Congress to enact the various laws that may apply to both the project development process and projects themselves. How the Corps is currently obligated to evaluate the environmental impacts of a project, as well as document and demonstrate compliance with any requirements that may apply as a result of those impacts, has evolved over many years. Several key requirements that require the Corps to plan and implement projects as it does currently were originally enacted in the 1960s and 1970s. It was during that time that national priorities and perspectives on the federal investment in water resources development were changing; at the time, public attention was turning to the impacts that human activities were having on the human and natural environment—that is, adverse impacts on air and water quality and on cultural and natural resources such as historic sites, plant and animal species, and/or their habitats. Beginning in the 1960s, Congress began to respond to that increased public awareness and concern by enacting various laws that affect project planning, development, approval, and funding, as well as laws intended to protect human health and the environment. Most requirements that currently apply to water resource project development that may be deemed "environmental" represent past efforts by Congress to minimize the potential for unforeseen adverse impacts and/or to mitigate or minimize any unavoidable adverse impacts. During the 1960s, Congress also turned its attention to the effects that federally funded projects were having on the environment—including Corps water resource projects. Generally, until the mid-1960s, analysis of water resource projects focused on the potential economic benefits and costs. With increased attention to adverse impacts of civil works projects, including the costs associated with remedying those impacts (see text box below), Congress enacted various laws that broadened Corps planning requirements to include an evaluation of project impacts to environmental quality. Of particular relevance to the Corps' planning process today was the enactment of the following: The Water Resources Planning Act of 1965 (P.L. 89-80)—created the Water Resources Council (WRC, now defunct due to lack of funding) and established "Water Resources Planning" requirements (42 U.S.C.