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to operate and maintain the gaging stations and to rate the measuring flumes on the ditches. The State Engineer is responsible for the distribution of water in the system to ensure the water is available at the time and place .of demand by water right owners who are in priority. His staff is also responsible for ensuring that the ratings on the ditches are kept current to ensure the proper amount of water is delivered to each ditch. Headgates and measuring flumes are required by statute on each diversion and the State Engineer has the authority to refuse water to the owners who fail to maintain these structures in proper order. In recent developments, most ofthe larger diversions have installed satellite-monitoring equipment, which allows the user as well as the State to acquire real-time data in order to ensure better administration. Hydrology of the Rio Grande and Conejos River The headwaters of the Rio Grande mainstem and the Conejos River are ringed by the Continental Divide. This area of southwestem Colorado normally receives a significant snowpack that provides the majority of the water that arrives at the upper index gages on the two rivers. These headwater areas are in relatively close proximity to the index gaging stations near Del Norte, Mogote, and Ortiz. Normally, the day's snowmelt or rain event runoff arrives at the gages during the next 12 to 24 hours, depending on what location in the basin one might consider. Since the operating reservoirs on both systems control only a fraction of the flow, the flows at the index gages are primarily a reflection of snowmelt or rainfall events. All these reservoirs hold relatively junior priorities and during the runoff, store under those decrees on a very limited basis when the flows at the index gages are very large. Therefore, during the irrigation season, the reservoirs bypass the inflow to them except for the highest portion of the runoff, if at all. Three ditches own the three irrigation reservoirs on the Rio Grande and the water from their decrees is not available to any. other ditches on the river. The Conejos Water Conservancy District, on the other hand, operates Platoro Reservoir and the water from it is available to the member ditches. It is a commonly held belief that all the irrigation reservoirs on the Rio Grande are available to all the ditches, or to store wa.ter for other purposes. This is obviously not the case and only the owners of the reservoirs can use the water available to them. Since Platoro is a post-compact reservoir, any water stored under its decree is accounted for as if it had passed the Mogote gage on a monthly basis. This stored water is then subtracted when it is released to ensure that the native water in the basin is properly accounted for and that the index supply and the corresponding obligation is not altered because of storage. The annual volumes of flow at the index stations are therefore relatively unaffected by the reservoirs on either of the Compact streams except on the occasion of a very wet year when some carryover can result. The hourly,. daily, seasonal and annual flows at the index stations are extremely variable. The daily diurnal effect during the runoff season as well as the variability of high altitude snowmelt can cause large changes within the day as well as from day to day. As is the situation with most western streams, the seasonal and annual flows are also highly variable. The past 25 years are a wonderful case study on variability of the water supply for the Rio Grande Basin. On the Rio Grande mainstem in Colorado, we have seen the historic low year in 1977 of 215,000 acre.feet and just a few years later saw three consecutive annual flows of over 1,000,000 acre-feet, a volume which has been exceeded only in seven of the 110 years ofrecorded history. The graph "Rio Grande River near Del Norte, CO - Annual Calendar Year Flows" shows the annual variability of streamflow at the Rio Grande near Del Norte gage. This gage is the upper index gage for the Rio Grande and is used to determine the amount of water owed to the downstream states, as well as the water available for distribution in priority to water rights owners. Peak flows on both systems are also reflective of the large variability of the low from year to year. On the Rio Grande near Del Norte gage, the peak averages around 5,400 cfs and varies over the history of the record from 1,730 cfs in 1977 to 18,000 cfs in 1912. The Conejos near Mogote gage shows a similar pattern with peak flows from 882 cfs in 1972 to 9,000 cfs;n 1912 with the average around 2,000 cfs. ,- :L. 5 >... -. v~~ ,~~;,,,,,j