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G3l,} . Streamflow traveltimes were estimated for river reaches using hourly streamflow data collected from 1990 to 1994 at four mains tern gaging stations [sites Ql, Q2, Q3, and Q4 (fig. 1)]. The estimates of reach traveltime were derived by compiling the time, in hours, for distinguishable streamflow peaks at the upstream site to arrive at the downstream site (peak-to-peak traveltime). Ungaged inflows were minimal during the intervals used for analysis. Streamflow gains and losses were evaluated using daily streamflow data at the upstream and downstream sites, available tributary data, and daily diversion data. Because gain-and-Ioss studies attempt to account for all inflows and outflows in a river reach, the time periods that contained some tributary data were used to characterize streamflow gains and losses. However, the amount of tributary data available to quantify surface-water inflows were limited and varied temporally and spatially in each reach. The two time periods that contained the most tributary data were 1957-67 and 1984-92. Streamflow data were available during these two periods at eight mainstem stations and seven tributary stations (table 1). Data from the mainstem station Purgatoire River at Ninemile Dam near Higbee [site Q3A (table 1)] were not used because of unresolved concerns about the quality of the station records (both time and stage) during some streamflow periods. The 1984-92 period was selected for most of the gain-and-loss analyses because this period had the best continuity of stream- flow data for the entire lower Purgatoire River, and this period represented the streamflow conditions that have existed along the lower Purgatoire River since 1979 when water regulation commenced at Trinidad Reservoir (Bureau of Reclamation, 1988). During 1984-92, there were few streamflow data available for the tributary streams and other side-channel inflows in reach 1. In reach 2, streamflow data for five gaged tributaries [sites T3- T7 (table I)] were used in the analysis, but data were not available for several other small ephemeral tributaries in reach 2. In reach 3, no streamflow data were available during any time period for the tributary streams and other side-channel inflows. Site Q4, which is located about 3.3 mi upstream from the mouth of the Purgatoire River, was periodically affected to some extent by surface-water return flows from the Las Animas Consolidated and Las Animas Consolidated Extension Canals that . receive water from outside the study area (Abbott, 1985). All surface-water outflows (diversions for irrigation) in the river reaches were measured and accounted for in the computation of daily streamflow gains and losses. Because the analysis of 1984-92 data for reach 1 did not result in quantification of streamflow gains and losses and because there were tributary data available for 1957-67, an analysis of gains and losses for a part of reach 1 was made from January 1957 through December 1967. Based on available data during this period, 34.2 mi of the Purgatoire River located between mainstem sites QIA and QIC was divided into two subreaches. The first subreach, reach lA (fig. 1), extended 10.8 mi from the Purgatoire River at Trinidad (site QIA) to the Purgatoire River near Hoehne (site QlB). Streamflow data for sites QIA and QlB (table 1) and diversion data for seven irrigation canals in reach 1 A were used in evaluating streamflow gains and losses. Most of the diverted water in reach lA was conveyed in unlined canals to cropland areas located farther downstream. There were several small ephemeral tributaries that might have contributed additional inflows to reach lA in response to rainfall. The second subreach, reach IB (fig. 1), extended 23.4 mi from site QIB to the Purgatoire River near Alfalfa (site QIC). Stream- flow data for sites QIB and QIC (table 1), for two gaged tributaries [sites T1 and T2 (table 1)], and diversion data for four irrigation canals in reach IB were used in evaluating streamflow gains and losses. No streamflow data were available for other ephemeral tributaries that might have contrib- uted additional inflows to reach lB. Because numerous tributary streams were ungaged along the lower Purgatoire River and because large errors can occur for a river reach in quantifying gains and losses when unmeasured inflows or outflows exist, the available tributary streamflow data for reaches I and 2 were used to evaluate whether ungaged tributary inflow affected estimates of stream- flow gains and losses. Method of Data Presentation Streamflow data and results of the analyses of streamflow gains and losses are illustrated in this report using boxplots. A boxplot (fig. 3) INTRODUCTION 7