Estimates of water demand in the Rock River Water Supply Planning Region (WSPR) were developed for the period 2010 to 2060. The estimates were developed separately for five major water demand sectors: (1) public supply; (2) self-supplied domestic; (3) self-supplied thermoelectric power generation; (4) self-supplied industrial and commercial; and (5) self-supplied irrigation, livestock, and environmental. Estimates were developed for all sectors on a county level and for public supply at a facility level for 42 dominant public systems, including the largest systems in each county. The techniques used to develop estimates differed by sector and included unit demand methods and multiple regressions. These methods provided estimates of future demand as a function of demand drivers and explanatory variables for many sectors and subsectors. Explanatory variables are those that influence unit rates of water demand, such as summer-season temperature and precipitation, median household income, marginal price of water, employment-to-population ratio, labor productivity, and precipitation deficits during the irrigation season. For most sectors and subsectors, total demand was estimated by multiplying unit rates of water demand by demand drivers. Demand drivers included such measures as population served by public systems, population served by domestic wells, number of employees, gross thermoelectric power generation, irrigated cropland acreage, irrigated golf course acreage, and head counts of various livestock types. For each sector, three scenarios were developed of future water demand that reflect different sets of plausible socioeconomic and weather conditions. These include a less resource intensive (LRI) scenario, a current trends (CT) (or baseline) scenario, and a more resource intensive (MRI) scenario. A “normal” climate, based on 1981-2010 climate “normals,” was assumed in all scenarios. Although the estimates suggest a plausible range of future demands, they do not represent forecasts or predictions nor indicate upper and lower bounds of future water demand. Different assumptions or different future conditions could result in predicted or actual water demands that are outside of this range.Total water demand in the Rock River WSPR was an estimated 1332 million gallons per day (Mgd) in 2010. Demand for self-supplied water for thermoelectric power generation dominates water demand in the region, making up 87 percent of the total water use, or about 1160 Mgd. Water for thermoelectric power generation is used almost entirely for cooling and generally returned to the source water body from which it was withdrawn, and thus is considered to be mainly non-consumptive. The consumptive loss, mainly in the form of evaporation, was estimated to be about 67 Mgd in 2010, or about 3.7 percent of the total. The CT and LRI scenarios assumed that regional gross thermoelectric power generation remains constant from 2010 to 2060, with no change in water demand. The MRI scenario assumed that one new thermoelectric plant having a gross capacity of 1200 MW with a closed-loop cooling system supplied with surface water would begin operations in Lee County in 2030. This would increase regional water demand for the thermoelectric power generation sector by 11 Mgd to 1171 Mgd.The second most important demand sector in the Rock River WSPR was public water systems, at 79 Mgd in 2010. Two counties accounted for more than 60 percent of the public water system demand, Winnebago County accounting for about 39 percent and Rock Island County about 23 percent. The irrigation, livestock, and environmental (ILE) sector was the next most important sector, with a demand of 52 Mgd in 2010, and most of this demand was for irrigation of cropland. Two counties, Whiteside and Lee, accounted for about 61 percent of the 2 irrigation demand in the region. The self-supplied industrial-commercial sector had a demand of 28 Mgd in 2010, with Rock Island County accounting for about half of this demand. The self-supplied domestic sector had the smallest demands, with 11 Mgd in 2010. Domestic demand was spread fairly evenly across the region, ranging from 0.4 Mgd (Lee County) to 1.6 Mgd (Ogle County). From 2010 to 2060, total demand in the region, not considering thermoelectric power generation, is estimated to decrease by 9 Mgd under the LRI scenario and increase 51 Mgd under the CT scenario and 141 Mgd under the MRI scenario. Most of the increase in total demand is accounted for by increases in self-supplied ILE demand, primarily for irrigated cropland. ILE demand is predicted to increase from between 7 Mgd (LRI) and 92 Mgd (MRI). The decrease in demand predicted by the LRI scenario is primarily due to decreasing demand (-16 Mgd) in the public supply sector. The sector totals for the thermoelectric power generation and industrial-commercial sectors are subject to revision, specifically, the simulation of new power plants and water-intensive industrial facilities as well as the retirement of existing facilities.Three climate change scenarios, ranging from hot/dry to warm/wet, were analyzed to determine the impact that increasing temperature and changing precipitation patterns could have on water demands. Public water system demands were calculated to increase between 6.0 and 8.7 percent because of climate change, and increases in domestic demands were similar. Irrigation demands varied from a decrease of 3.2 percent in a wetter future environment to an increase of 10.1 percent in a drier environment. The impact of periodic droughts was also examined. For a severe drought, public water system demand was calculated to increase by 8.7 percent and cropland irrigation demand by 34.0 percent. Demands would return to normal once the drought ended.