Cooperative Watershed Management Act of 2008

Cooperative Watershed Management Act of 2008 - Directs the Secretary of the Interior to establish a Cooperative Watershed Management Program, under which the Secretary shall provide grants to form or enlarge a watershed group or to conduct water management projects in accordance with the goals of such a group.

Defines a "watershed group" as a self-sustaining, cooperative watershed-wide group that: (1) is comprised of representatives of the affected stakeholders of the relevant watershed; (2) is a grassroots, nonregulatory entity that addresses water availability and quality issues within the relevant watershed; (3) is capable of promoting the sustainable use of the water resources of the relevant watershed and improving the functioning condition of rivers and streams through water conservation, improved water quality, ecological resiliency, and the reduction of water conflicts; and (4) makes decisions on a consensus basis. Requires a watershed group to include: (1) representatives of hydroelectric production, livestock grazing, timber production, land development, recreation or tourism, irrigated agricultural production, the environment, potable water purveyors and industrial water users, and private property owners within the watershed; (2) any federal, state, or local agency agency that has authority regarding the watershed; and (3) any Indian tribe member who owns land within the watershed or land held in trust.

Authorizes the Secretary, in distributing grant funds, to give priority to watershed groups that represent maximum diversity of interest or serve subbasin-sized watersheds with an eight-digit hydrologic unit code. Sets forth usage and progress requirements for, procedures for awarding, and federal shares of the costs of first-, second-, and third-phase grants.

Requires: (1) annual reports by watershed groups to the Secretary on progress; and (2) reports every five years by the Secretary to specified congressional committees on Program benefits and effectiveness.

Authorizes appropriations for FY2008-FY2020.