Estimating Tribal Reservation Areas and Population without CWA WQS
Purpose
To estimate the geographic extent and population of tribal reservation lands without applicable Water Quality Standards (WQS) under the Clean Water Act (CWA) ("applicable WQS").
Methodology
For this analysis, EPA defines "reservation lands" to be lands within the boundaries of reservations and off-reservation trust lands of federally recognized Indian tribes.  
EPA defined the following metrics for this analysis:
                                    Metric
                     Total within tribal areas (Universe)
River and stream miles in the National Hydrography Dataset (NHD) that are within the boundaries of American Indian Off-reservation Trust Lands and American Indian Reservations represented by the EPA Tribes (3 of 6): Trust Lands and EPA Tribes (6 of 6): American Indian Reservations data layers published by the U.S. EPA Office of Information Management.
                                 122,576 miles
Square miles of lakes, impoundments, and other open waters in the NHD that are within the boundaries of American Indian Off-reservation Trust Lands and American Indian Reservations represented by the EPA Tribes (3 of 6): Trust Lands and EPA Tribes (6 of 6): American Indian Reservations data layers published by the U.S. EPA Office of Information Management.
                              3,654 square miles
Resident 2010 population of geographic areas identified by the U.S. Census as "American Indian Reservation and Off-Reservation Trust Land  -  Federal" in Table 45 of a 2012 report ("2012 Census Report").
                                    947,406

For the above metrics, EPA excluded all waters and population associated with the 38 federally recognized tribes in the State of Oklahoma. Under federal law unique to Oklahoma, state WQS are currently applicable under the Clean Water Act for most tribal reservation lands. However, the absence of consistent geospatial and population information prevents systematic analysis of the areas where state WQS do not apply, which include tribal trust lands, individual Indian allotments, and certain Indian-owned fee lands. 
Next, for each metric EPA identified the tribal areas containing waters to which WQS were applicable under the Clean Water Act as of March 2023. Specifically, EPA identified the following:
 Reservation lands for which EPA has approved the WQS of an authorized tribe pursuant to sections 303(c) and 518 of the Act. There were 47 such tribes as of March 2023. 
 Reservation lands for which EPA has promulgated federal WQS. There was one such tribe (the Confederated Tribes of the Colville Reservation in Washington) as of March 2023.
 Reservation lands in certain states for which Congress has granted jurisdiction to administer state WQS. These include:
 Reservation lands of four tribes in Maine: Aroostook Band of Micmacs, Houlton Band of Maliseet Indians, Passamaquoddy Tribe, and Penobscot Nation.
 Reservation lands of the Catawba Indian Nation in South Carolina.
 Non-Indian fee lands of the Puyallup Reservation in Washington. EPA-approved tribal WQS apply on waters of other lands within the Reservation. 
Finally, EPA determined the figures for each metric related to tribes with CWA-applicable WQS and tribes without CWA-applicable WQS. 
Results
                                    Metric
                               Reservation areas
                                     with
                              CWA-applicable WQS
                           Reservation areas without
                              CWA-applicable WQS
                            Total reservation areas
Tribes with reservation areas 
                                   53 tribes
                                  243 tribes
                                  296 tribes
Rivers and steams
                                 47,055 miles
                                 75,521 miles
                                 122,576 miles
Lakes, impoundments, and other open waters
                               726 square miles
                                (464,640 acres)
                              2,928 square miles
                               (1,873,920 acres)
                              3,654 square miles
                               (2,338,000 acres)
Resident population
                                    415,335
                                    532,071
                                    947,406

Uncertainties
This analysis does not include certain tribal trust lands in Oklahoma, Alaska, and possibly elsewhere for which geospatial and population information is not consistently available but which currently lack WQS applicable under the Clean Water Act. 
For tribes with EPA-approved tribal WQS, the analysis generally assumes that the approved WQS apply to waters in all reservation lands of the tribe, even though in some cases the tribe has not adopted, and EPA has not approved, WQS for all such waters. 
The reservation areas in the EPA Tribes (3 of 6): Trust Lands and EPA Tribes (6 of 6): American Indian Reservations data layers and the 2015 Census Report may be incomplete or have unknown errors in representing the boundaries or legal status of formal reservations and tribal trust lands. For example, some recently established tribal trust lands may have been taken into trust after the Data Layer and Census Report were finalized. 
Note that the analysis did not use certain population sets in the 2012 Census Report (pp. 301-307) that were not relevant to the Baseline WQS rule.  
References
A fact sheet on EPA's TRIBES services. https://www.epa.gov/sites/default/files/2020-05/documents/tribal_identification_services_fact_sheet_may_2020.pdf.
EPA's Tribal Identifier Data Standard, https://www.epa.gov/data-standards/tribes-services-tribal-identifier-data-standard#file-150427. 
McKay, L., Bondelid, T., Dewald, T., Johnston, J., Moore, R., and Rea, A., NHDPlus Version 2, 2012. https://www.epa.gov/waterdata/nhdplus-national-data. 
U.S. EPA Office of Information Management. EPA Tribes (3 of 6): Trust Lands. October 29, 2021. https://edg.epa.gov/metadata/catalog/search/resource/details.page?uuid=%7BAA7C6238-924B-4F49-93B8-EAD2713A11FD%7D.  
U.S. EPA Office of Information Management. EPA Tribes (6 of 6): American Indian Reservations. October 29, 2021. https://edg.epa.gov/metadata/catalog/search/resource/details.page?uuid=%7BC0C4D9E8-DFC1-42FF-B53C-55E454A38D03%7D.
U.S. Census Bureau, 2010 Census of Population and Housing, Population and Housing Unit Counts, CPH-2-1, United States Summary, U.S. Government Printing Office, Washington, DC, 2012. Available at https://www.census.gov/library/publications/2012/dec/cph-2.html. 

