This study was undertaken to improve the understanding of water resources of the Tulalip Plateau area, with a primary emphasis on the Tulalip Indian Reservation, in order to address concerns of the Tulalip Tribes about the effects of current and future development, both on and off the Reservation, on their water resources. The drinking-water supply for the Reservation comes almost entirely from ground water, so increasing population will continue to put more pressure on this resource. The study evaluated the current state of ground- and surface-water resources and comparing results with those of studies in the 1970s and 1980s. The study included updating descriptions of the hydrologic framework and ground-water system, determining if discharge and base flow in streams and lake stage have changed significantly since the 1970s, and preparing new estimates of the water budget. The hydrogeologic framework was described using data collected from 255 wells, including their location and lithology. Data collected for the Reservation water budget included continuous and periodic streamflow measurements, micrometeorological data including daily precipitation, temperature, and solar radiation, water-use data, and atmospheric chloride deposition collected under both wet- and dry-deposition conditions to estimate ground-water recharge. The Tulalip Plateau is composed of unconsolidated sediments of Quaternary age that are mostly of glacial origin. There are three aquifers and two confining units as well as two smaller units that are only localized in extent. The Vashon recessional outwash (Qvr) is the smallest of the three aquifers and lies in the Marysville Trough on the eastern part of the study area. The primary aquifer in terms of use is the Vashon advance outwash (Qva). The Vashon till (Qvt) and the transitional beds (Qtb) act as confining units. The Vashon till overlies Qva and the transitional beds underlie Qva and separate it from the undifferentiated sediments (Qu), which are also a principal aquifer of the plateau. The undifferentiated-sediments aquifer is present throughout the entire study area, but is not well defined because few wells penetrate it. Ground water flows radially outward from the center of the Plateau in the Vashon advance outwash aquifer. Water levels fluctuate seasonally in all hydrogeologic units in response to changes in precipitation over the course of the year. However, water levels do not appear to have changed significantly over the long term. There was no statistically significant change between water levels measured in 72 wells in the early 1990s and 2001. Additionally, when a rank sum test was used to compare monthly water levels measured in 18 wells for this study with monthly water levels from the 1970s and 1980s, water levels increased in some wells, decreased in some, and did not change significantly in others. Ground water in the study area is recharged from precipitation that percolates down from the land surface. Average annual recharge, estimated using the chloride-mass-balance method, was 10.4 inches per year. Current streamflow conditions on the Reservation were defined by four continuous-record streamflow-gaging stations operated from April 2001 through March 2003 and monthly measurements of discharge at 12 periodic-measurement sites. Two continuous-record gaging stations (12157250 and 12158040) near the mouths of Mission and Tulalip Creeks, respectively, also were operated during water years 1975-77. Correlations of streamflow for Mission and Tulalip Creeks with the long-term record of streamflow at Mercer Creek (station 12120000) indicate no significant change in streamflow between the mid-1970s and 2001?03 in Mission and Tulalip Creeks. However, comparisons between the percentage of change in precipitation at the Everett precipitation station and percentages of change in streamflow at the Mercer, Mission, and Tulalip Creek gaging stations from the mid-1970s through 2001