Purpose and Scope

The purpose of this report is to describe a preliminary analysis of the hydrologic dynamics related to irrigation diversions and return flow in the Upper Gunnison River Basin, specifically in a selected reach of the East River, Colo. (fig. 1). This report compiles and analyzes published streamflow data from two streamgages and relevant irrigation diversions. The data were analyzed by mass balance to quantify the net gains and losses of water within the study reach on daily and annual time steps. Consumptive loss of water by evapotranspiration was not directly assessed, nor was movement of groundwater into or out of the study reach. The report does not address processes that affect water balance and does not directly assess return flows. The understanding presented here is a step to facilitate additional sophisticated analysis and modeling of local hydrologic dynamics in the basin.

Study Area

The stream reach studied is a part of the East River between U.S. Geological Survey streamgage 09112200 (East River below Cement Creek near Crested Butte, Colo.) and U.S. Geological Survey streamgage 09112500 (East River at Almont, Colo.), which is located downstream from 09112200 (fig. 1). The study reach has a direct contributing area of 50 square miles (mi²; U.S. Geological Survey, 2024) and a contributing area of 239 mi² upstream from the upper bounding streamgage (09112200). The straight-line distance between the streamgages is about 8.2 miles. The upstream contributing area contains a larger amount of higher elevation terrain than the direct contributing area and therefore yields more snow-derived streamflow. Inflows from tributaries within the study reach are relatively small compared to inflow from upstream. Land irrigated by diverting water from the East River and its tributaries accounts for 5.23 mi², or roughly 10 percent, of the contributing area between the two streamgages (Colorado Division of Water Resources, 2021).

The hydrographs of rivers in the region are dominated by snowmelt from April through July, have brief pulses of runoff from monsoonal storms from July into the fall, and have lower flows through the rest of the year (U.S. Geological Survey, 2024). Irrigation in the study reach is generally accomplished by diverting water from the East River and its tributaries through ditches. The Slide Ditch (fig. 1) diverts notable quantities of water immediately upstream from the U.S. Geological Survey streamgage (09112200) at the upstream end of the study reach (Colorado Division of Water Resources, 2023). Apart from the Slide Ditch, there are small to negligible effects on streamflow from ditches, pastures, or local geography conducive to substantial shallow aquifer flow that would potentially convey water past the streamgages that constitute the upstream and downstream boundaries of the study reach. However, two other ditches that could complicate the boundaries are specifically considered in the section of this report titled “Flows Excluded from the Analysis.” There are 19 irrigation diversions and associated ditches that are entirely internal to the study reach, meaning that their inflow and outflow do not cross the boundaries of the reach (fig. 1). Aside from irrigated pastures, most of the land is undeveloped and forested, with some residential use. Application of irrigation water from ditches in the study reach is generally accomplished by diverting water from primary irrigation ditches and smaller laterals without pipes or infrastructure into pastures that generally have not been furrowed, leveled, or otherwise modified. This irrigation type is colloquially known as “wild flood.”

U.S. Geological Survey streamgage 09112200 (East River below Cement Creek near Crested Butte, Colo.), hereafter the “upstream streamgage,” is at 8,440 feet above National Geodetic Vertical Datum of 1929 (NGVD 29) and has a contributing area of 239 mi² that contains elevations greater than (>) 13,000 feet (U.S. Geological Survey, 2024). About 240 yards upstream from this site is the diversion point for the Slide Ditch (Colorado Division of Water Resources site 5900672 ), which diverts water from the East River to pastures along the east side of the river (Colorado Division of Water Resources, 2024). The U.S. Geological Survey streamgage 09112500 (East River at Almont, Colo.), hereafter the “downstream streamgage,” is at 8,006 feet above NGVD 29 and has a contributing area of 289 mi² (U.S. Geological Survey, 2024). The intervening study reach contains elevations greater than 12,000 feet.

Two other ditches were considered for their relevance to main stem inflows to and outflows from the study area because they bypass the streamgages that bound the study reach (Colorado Division of Water Resources, 2024). The Imobersteg Ditch diverts water out of the East River on the west side about 2.4 miles upstream from the upper study area boundary. The Imobersteg Ditch is 3.3 miles and irrigates a pasture approximately 1.0 mile long directly west of the upstream streamgage and does not connect back to the East River. The Marston Ditch diverts water from the East River just upstream from the lower study area boundary, is 0.9 miles, and irrigates a narrow area of 0.6 miles next to the river. Diverted water not used for irrigation is returned to the East River about 50 yards downstream from the downstream streamgage.

Data Sources

Data from three monitoring locations, two bounding the study area upstream end and one bounding the study area downstream end, constituted the core data used in the study (fig. 1). Daily mean streamflow data from the two U.S. Geological Survey streamgages were used in the analyses; the daily mean streamflow values (hereafter “daily streamflow”) were means based upon measurements made at 15-minute intervals from 1994 to 2023 (U.S. Geological Survey, 2024). Annual total streamflow was calculated as the sum of the daily streamflow for each year. Total annual flow for a specified period was calculated as the sum of the annual total streamflows.

Streamflow data for all ditches were available on a daily time step, but the values were based upon manual measurements made during site visits. The manual measurements were recorded as identical values for each subsequent day until the next manual measurement (Colorado Division of Water Resources, 2024). The schedule for site visits varied by site. Typical return intervals were about 30 days but were sometimes more frequent, particularly if there was a substantial change in flow. Data used in this report were the daily values as recorded. All sites had a complete data record spanning water years 1994–2023. A water year is the 12-month period from October 1 through September 30 of the following year and is designated by the calendar year in which it ends.

Within the study reach boundaries were 19 irrigation diversions, with 14 ditches that diverted water from the East River and 5 ditches that diverted water from tributaries (Colorado Division of Water Resources, 2024). As these ditches were internal to the study reach, meaning that they did not cross the study reach boundaries, they were not used in the mass balance analysis but were useful for understanding volumes of water diverted and used for irrigation. Because monitoring was infrequent, there was substantial uncertainty regarding flows in these ditches and the other ditches described in the “Study Area” section of this report.

Mass Balance Analysis

Streamflow data for the upstream and downstream streamgages and Slide Ditch were tabulated and converted to total flows in acre feet (acre-ft) on an annual basis. The same was done for the 19 ditches that do not cross the boundaries of the study reach. Because most diversions continued until October 31, annual total flows were determined for irrigation years from November 1 through October 31. The mass balance analysis compared main stem inflows and outflows for the study reach. Annual total streamflows at the upstream streamgage and the Slide Ditch were combined to determine total annual main stem inflows (hereafter “combined inflows”). The combined inflows were subtracted from outflows at the downstream streamgage to determine annual net gain or loss of water in the study reach. The annual net gains or losses were assessed to determine how the broader hydrologic dynamics have changed through time. With the derived understanding, two general hydrologic conditions for the study reach (general deficit and general surplus, discussed in the section titled “Annualized Mass Balance Analysis”) were identified, each spanning multiple years.

Summary statistics (medians and 5th and 95th percentiles) were calculated for daily mean streamflow and net gains and losses within the time periods associated with each hydrologic condition. Using these summary statistics, common patterns in the timing of streamflow gains and losses within the reach could be discerned. Diversions and return flows can have a major, though not the only, affect on gains and losses of streamflow; therefore, the magnitude and timing of peaks in streamflow gains and losses were assessed from the daily summary statistics for the two hydrologic conditions.

Flows Excluded from the Analysis

The Imobersteg Ditch, which diverts water from the East River upstream from the upstream streamgage (fig. 1), lay largely outside the study reach. However, return flow from the ditch could have contributed inflows of water to the study reach, and the potential magnitude of those inputs was considered relative to combined inflows from the East River and Slide Ditch. From 1994 to 2023, annual total flows in the Imobersteg Ditch averaged 2,420±999 acre-ft (mean±standard deviation) with a range of 920–4,064 acre-ft (Colorado Division of Water Resources, 2024). As a percentage of the combined inflows per year, annual total flows in the Imobersteg Ditch averaged 1.3±0.8 percent with a range of 0.2–3.4 percent. Evapotranspiration likely consumed a large but unknown quantity of that diverted water because crop transpiration is the purpose of irrigation. Therefore, the Imobersteg Ditch was excluded from the mass balance analysis.

The Marston Ditch diverts water away from the East River from within the study reach and returns unused water back into the East River at the ditch outlet downstream from the downstream streamgage (fig. 1). Therefore, return flows in the Marston Ditch are not accounted for by the downstream streamgage and need to be considered. From 1994 to 2023, annual total diversions by the Marston Ditch averaged 2,337±819 acre-ft with a range of 247–4,515 acre-ft (Colorado Division of Water Resources, 2024). As a percentage of annual total streamflows at the downstream streamgage, diversions by the Marston Ditch averaged 1.3±0.9 percent with a total range of 0.1–4.6 percent. A large quantity of that diverted water likely evapotranspired, and subsurface return flows from Marston Ditch likely discharged to the stream upstream from the downstream streamgage. Therefore, the Marston Ditch was excluded from the mass balance analysis.

Annualized Mass Balance Analysis

From 1994 to 2023, annual total streamflow measured at the upstream streamgage averaged 219,436 acre-ft (range 100,056–387,686 acre-ft; U.S. Geological Survey, 2024). Annual total flow in the Slide Ditch averaged 4,148 acre-ft (range 1,542–7,050 acre-ft; Colorado Division of Water Resources, 2024). Combined inflows averaged 223,584 acre-ft (range 104,417–393,896 acre-ft; fig. 2A). Therefore, the Slide Ditch averaged 1.9 percent of combined inflows (range 0.06–4.5 percent). Outflow at the downstream streamgage averaged 222,863 acre-ft (range 97,334–418,263 acre-ft; fig. 2A). Combined inflows and outflow were therefore similar in magnitude. Irrigation diversions within the study reach are a useful point of reference. Total diversions within the study reach averaged 48,667 acre-ft (range 32,194–61,907 acre-ft) and therefore averaged 25 percent (range 8–55 percent) of combined inflows, though it is important to note that some of those diversions occurred on tributaries (fig. 2B).

Annual net gains and losses of streamflow within the study reach were computed by subtracting combined inflows from the outflow. Annual streamflow in the study reach showed both net loss (negative values) and net gain (positive values) depending upon the year (fig. 2C). Annual net gains were likely from precipitation within the study reach that exceeded actual evapotranspiration to yield net natural runoff. Years with annual net losses could have been caused by consumption of diverted water by evapotranspiration and or movement of that water into storage, particularly in shallow aquifers, in magnitudes that exceeded natural net runoff within the study reach. Annual net gains and losses were generally small and averaged −721 acre-ft, with a range of −24,040–26,218 acre-ft (fig. 2C). As a percentage of inflows for the irrigation year, annual net gains and losses averaged −0.8 percent and ranged from −10.2 to 8.9 percent. Annual net gains and losses varied from year to year with an indication of an upward trend from about 1997 through 2024 (fig. 2C). By adding the annual net gain or loss for each subsequent year to the total from previous years, the cumulative gain or loss for the study reach was calculated (fig. 2D). Using this information, two generalized hydrologic conditions were identified. The first hydrologic condition, hereafter referred to as “general deficit,” was generally represented by a net loss or consumption of streamflow within the study reach (fig. 2D). This condition lasted for a 16-year period (1997–2012). The second hydrologic condition, hereafter referred to as “general surplus,” was generally represented by a net gain of streamflow within the study reach and lasted for a 10-year period (2014–23; fig. 2D).

In terms of understanding the two hydrologic conditions, there was no obvious pattern in East River streamflow that could seem to correlate directly with the timing (fig. 2A, D). Lower flow years were more common during the general deficit but also occurred during the general surplus. Higher flow years were more common during the general surplus but also preceded the general deficit hydrologic condition (1995–96) and were present towards its end (2008 and 2011; fig. 2D). Patterns in evapotranspiration also seemed an unlikely explanation for the shift in hydrologic conditions because it would take a downward trend in evapotranspiration to favor gains in streamflow. However, in the broader Upper Colorado River Basin, evapotranspiration has been trending upward (Milly and Dunne, 2020). Additionally, evapotranspiration from irrigated pastures could have been expected to correlate positively with diversions, and there is no indication of a downward trend in diversions (fig. 2B).

In contrast, changes in excess storage capacity in the shallow aquifer in the study reach could have produced the pattern observed. Flood irrigation can enhance the recharge of shallow aquifers as water percolates below the rooting zone (Gordon and others, 2020). If annual recharge due to irrigation exceeded the rate at which water could move laterally in the shallow aquifer to discharge to surface water, a rise in the water table could have been the response. If water tables generally rose each year, the difference in hydraulic head between shallow aquifers fed by irrigated pastures and surface water channels could have increased, driving a proportional increase in the lateral velocity of groundwater moving through the subsurface. As the rising water table approached the surface, subsurface flow paths to surface discharge locations could have become shorter, which, coupled with increased groundwater flow velocities, could have increased rates of groundwater discharge into surface water. In locations where higher water tables generally filled aquifer storage capacity, infiltration could not have been possible, and irrigation water could have run over the surface as increased tail water flows.

It is possible that a portion of the net losses during general deficit years reflect the filling of available groundwater storage capacity with infiltrated irrigation water. Some groundwater travel times from the point of infiltration to discharge to surface water would have had to exceed one year for such filling to occur; otherwise, net losses by way of increased storage would not have been detected annually. A scenario where excess capacity in the shallow aquifer decreased during multiple years, until excess storage capacity was generally eliminated, is a potential explanation for the change in hydrologic condition from a generalized deficit to a generalized surplus and could also relate the changes in hydrologic condition to irrigation and return flows. However, determining the specific causes for the two hydrologic conditions, and what caused the shift from one to another, was beyond the scope of this report.

Daily Mass Balance Analysis

The medians of daily mean streamflow values, along with the daily 5th and 95th percentiles, for net gain or loss of daily mean streamflow within the study reach were compared between the time periods representing the two previously defined hydrologic conditions—general surplus and general deficit (fig. 3A, B). This analysis revealed when and how much water was typically lost or gained during the irrigation year during the two different hydrologic conditions. It could be assumed that streamflow losses were generally related to diversions and consumption of diverted water by evapotranspiration. It also could be assumed that streamflow gains were generally related to return flows and natural runoff from precipitation and snowmelt. Examination of median daily gains and losses revealed broad patterns that could have been difficult to discern in individual years. The analysis supported the concept of the hydrologic conditions and revealed how they may have affected return flow dynamics.

Patterns during winter may reflect different groundwater conditions during the two hydrologic conditions (fig. 3A, B). Variations were generally small from November through March, a time when diversions were not occurring (Colorado Division of Water Resources, 2024) and subfreezing temperatures limited runoff. During general deficit, gains during winter were commonly around 4.6 cubic feet per second (ft³/s), and during general surplus, they were commonly greater at around 7.6 ft³/s, a difference of 3 ft³/s or 65 percent. As winter base flow discharge is understood to be driven by groundwater discharge, the greater gains were consistent with increased shallow groundwater storage during general surplus.

The growing and irrigation season patterns (approximately April through August) were where shorter-term return flow dynamics were assumed to be apparent (fig. 3A, B). The median statistics provided a representation of flow timing and magnitude that dampened variation that was much more substantial in individual years. In mid-April, streamflow usually began to increase substantially most likely in response to snowmelt, and by early May, irrigation diversions generally began. During general deficit, daily net losses (losses) of streamflow commonly began around this time. Net losses commonly reached their extreme for the year around May 27 at −110 ft³/s (fig. 3A). On average, daily net gains (gains) peaked 73 days later, on August 8, at a median gain of 24 ft³/s, at which time snowmelt runoff had usually faded and median flows in the East River were about 230 ft³/s (U.S. Geological Survey, 2024). The 5th and 95th percentiles indicate that streamflow losses less than −100 ft³/s were not unusual, but gains greater than 50 ft³/s were unusual.

During general surplus, the patterns were more complex (fig. 3B). There was commonly a peak in gains before there were substantial losses. The first broad period of gains commonly peaked around May 24 at around 12 ft³/s, during the ascending limb of snowmelt runoff when East River streamflow was greater than 1,000 ft³/s (U.S. Geological Survey, 2024). This was followed by two pairs of loss minimums and peaks in gains. The first loss minimum occurred on June 6 at −37 ft³/s and was followed by a broad peak in gains that commonly occurred around June 24 at 55 ft³/s. This peak occurred when streamflow in the East River was commonly on the descending limb of the hydrograph relative to snowmelt runoff but still with median daily streamflows greater than 1,000 ft³/s (USG U.S. Geological Survey S, 2024). Another local minimum for losses occurred around July 20 but was nominal at −3 ft³/s. A final peak in gains occurred around August 11 at 35 ft³/s. The second pair of minimum and peak occurred when streamflows were generally less than 400 ft³/s (U.S. Geological Survey, 2024) and entering the long tail of snowmelt runoff. The times between pairs of local minimums for losses and peaks for gains were 18 and 22 days, respectively.

The patterns in median daily gains and losses likely reflect return flow dynamics during the two hydrologic conditions. During the general deficit, the substantial diversions of water from the East River and its tributaries resulted in notable net loss of streamflow in May and June (fig. 3A). The modest peak in gains 75 days later most likely reflected the typical average time it took for excess irrigation water to reach the East River. This length of time suggests water likely moved through longer subsurface flow paths and interacted with excess capacity in the shallow aquifer. During the general surplus, net losses were much smaller, net gains were much larger, and the times between them were much shorter than during the general deficit (fig. 3B). Such patterns most likely indicated little capacity to absorb excess irrigation water in the shallow aquifer and the forcing of excess water into shorter, faster flow paths in the subsurface or over the surface as tail water.

Comparisons of the median daily net gains and losses to streamflow at the downstream end of the study reach offer perspective on how irrigation and return flow dynamics affect the East River. During general deficit, net losses occurred during peak snowmelt runoff, and a net loss of greater than 50 ft³/s could be considered small compared to concurrent streamflow that exceeded 1,000 ft³/s (fig. 4A, B). In the condition of general surplus, the fluctuations in gains and losses were even smaller compared to streamflow (fig. 4A, B). The percentages of daily net gains and losses relative to streamflow were generally within 10 percent, regardless of the hydrologic condition of the study reach (fig. 4C). Thus, effects of irrigation and return flow dynamics on the study reach have only modest effects on volumes of streamflow in the East River. However, similar dynamics may have occurred or be occurring in other areas with irrigated agriculture in the Upper Gunnison River Basin and elsewhere. When such dynamics are scaled to larger areas that involve larger volumes of water, even modest changes in irrigation and return flows could have a substantial cumulative effect on water dynamics and availability in the Gunnison River Basin. The results of this study suggest that the timing and magnitudes of return flow dynamics are likely strongly affected by antecedent conditions in local shallow aquifers. That insight could be useful to subsequent studies that seek to understand the potential effects of irrigated agriculture and return flows in the region.