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. It in diameter (1982 Lawn Lake dam-failure flood in the Roaring River). Regional Analysis of Streamflow Data A lack of flood evidence in one basin such as Elkhead Creek Basin could result from pure chance, Thus, it is essential to ascertain the flood history for other basins in a region, Regional analysis is concemed with extending records and providing a tool to estimate streamflow values at ungaged sites (Wahl, 1984). In addition, regional analyses produces improved estimates of streamflow characteristics at gaged sites by decreasing time-sampling errors. The existence of the upper limit to the magnitude of floods in a region is a long standing issue in flood hydrology (Enzel and others, 1993), Envelope curves encompassing the maximum flood peaks in a homogeneous hydrometeorologic region have long been used in flood hydrology (Crippen and Sue, 1977; Wolman and Costa, 1984; Jarrett, 1990b; Enzel and others, 1993; Jarrett and others, in review). Utilization of envelope curves for a hydrometeorologic region assumes that the maximum flood in a basin is likely to be experienced in a nearby basin. However, not all basins in the region will have the maximum flood, rather no basin has had a flood that exceeds the envelope curve for the specifiC region. The primary limiting factors in extreme-flood production are amount, intensity, duration, and spatial distribution of rainfall, orographic enhancement of rainfall and basin slope (Costa, 1987b). Wolman and Costa (1984) and Enzel and others (1993) indicated that with increasing documentation of flood and paleoflood data the envelope curves have essentially stabilized. Thus, they . concluded the stability in the envelope curves adds a new level of confidence to the method, particularly when paleoflood data are included. Jarrett (1994) indicaled one reason for stability in envelope curves is that historical, extreme floods primarily were overestimated in mountain rivers due to underestimating flow- resistance coefficients by an average of about 60 percent, thus, flood estimates were overestimated by a comparable amount. Jarrett (1987, 1994), Costa (1987a), and Enzel and others (1993) indicated that if a flood estimate exceeds the envelope curve for the region, then the flood calculations, particularly energy losses, need to be reassessed, to ascertain the accuracy of the discharge estimate. Clearly, extreme floods have the greatest implications in design of hydraulic structures located in floodplains and for flood- plain management. Streamflow data in the Yampa and White River Basins upstream from an elevation of about 5,235 It in northwestem Colorado were evaluated to assess regional bounds of extreme flooding, This part of the Yampa and White River Basins is a homogeneous hydrometeorologic flood region (Miller and others, 1973; McCain and Jarrett, 1976; Kircher and others, 1985; Jarrett, 1987). A retrieval of streamflow-gaging station peak-flow data through 1995 was done to update the envelop curves of maximum unit discharge versus elevation for Colorado (Jarrett, 1987, in review). . 17