Grand Valley Flow Trends Analysis

Introduction

The Grand Valley hosts two major rivers along with several canal systems and municipal users that divert water directly from the river and distribute water it across the valley. This analysis considers together river flow data at U.S. Geologic Survey (USGS) to evaluate seasonal and annual flow magnitudes and frequencies. That is, how often do we expect to see a certain flow rate during a certain season.

In this analysis, we primarily consider trends in annual flow metrics, that is: are certain aspects of the flow regime increasing or decreasing over time? The Upper Colorado River Basin Fish Recovery Program has established annual and monthly high and low flow targets for the 15-Mile Reach (Palisade to the Colorado-Gunnison Confluence). We rely on these flow targets to inform some of our trend analysis below.

Data and Flow Metrics

The following data sources (average daily values) are used:

Station Name	Station ID	Start Date	End Date	Years
COLORADO RIVER NEAR CAMEO, CO.	09095500	1933-10-01	2025-12-31	92
PLATEAU CREEK NEAR CAMEO, CO	09105000	1936-04-26	2025-12-31	88
COLO RIVER BELOW GRAND VALLEY DIV NR PALISADE, CO	09106150	1990-10-01	2025-12-31	35
GUNNISON RIVER NEAR GRAND JUNCTION, CO.	09152500	1896-10-01	2025-12-31	117
COLORADO RIVER NEAR COLORADO-UTAH STATE LINE	09163500	1951-05-01	2025-12-31	75

The following annual flow metrics are calculated for each of the USGS stream gages referenced above.

Metric	Symbol	Units
Mean Annual Discharge	MAD	CFS
Median Annual Discharge	MED	CFS
Maximum 7-day Discharge	MX7D	CFS
Minimum 7-day Discharge	MN7D	CFS
Total Volume	VTOT	KAF
Day of Year 25% Volume	V25	DAY
Day of Year 50% Volume	V50	DAY
Day of Year 75% Volume	V75	DAY
Day of Year 90% Volume	V90	DAY
Max 7-day Day of Year Anomoly	MX7DA	DAY

The following plots summarize the above annual discharge metrics displayed as time series to indicate trends.

Annual average and median flow show annualized trends in average and total flow for each stream gage. We see slight downward trends in average flow (and runoff volume) as indicated by the local trend line (solid line in each plot), which indicates the overall trend of the data. Note that a statistical trend analysis has not been performed. These lines indicate visual trends.

The Maximum 7-Day Average Flow shows a downward trend for Colorado River at Cameo and Plateau Creek. It appears to be relatively stable for the Gunnison River and the Colorado River at the Utah state line. This means that high (peak) flows are decreasing for the two upstream gages but are relatively stable for the Gunnison and Colorado Rivers downstream of the Grand Valley.

The Minimum 7-day average flow values are decreasing across all stream gages. This means that low flows are becoming lower.

The timing (day of year) of the maximum 7-day average within each year is compared to the average day of year of the maximum 7-day period from the baseline time period of 1971-2000. This time period captures runoff years prior to the extended drought that began in the 21st century (2000 to present). The timing of this annual peak 7-day period appears to be relatively stable for all gages except the Gunnison River. A negative trend in the timing of the peak indicates an earlier peak compared to the reference period.

Trends in Variability in Annual Flow Metrics

In addition to trends in flow metrics, trends in variability of these metrics is also of interest. If variability (greater spread between high and low years about the mean) is changing over time, this indicates a change in the expected extreme values.

In the figures below, the standard deviation of selected flow metrics is calculated based on a 10-year moving window. The value for a given year represents the standard deviation of the metric based on the 10-year window ending at that year.

For the majority of the gages below, variability peaks in the 1990s and has reduced to a local minimum in that last decade. One exception is the day of the year at which maximum seven day period occurs. Variability in this metric has increased.

Water Year Types and Trends

The US Fish and Wildlife Service - Upper Colorado River Basin Endangered Fish Recovery Program (Recovery Program) evaluates the “runoff year type” based on the period of record percentiles of April to July runoff volume. They define four runoff year classes as follows:

Runoff Year Class	Apr-Jul Vol Percentile
Wet	\(\geq\) 75%
Avg-Wet	< 75%
Avg-Dry	< 50%
Dry	< 20%

Runoff from the USGS gage at the Colorado River at Cameo is classified here using the same percentile groupings based on the entire period of record. We plot the April to July runoff volumes for the Cameo gage and classify them based on the percentiles from these different periods.

It can be seen from above that the frequency of “Dry” years has increased in the last several decades. Also, recent “Wet” years have just barely exceeded the percentile threshold for classifying a runoff year as wet.

Probability of Successive Dry Years

In the previous figure, it is evident that the frequency of dry or low runoff years has become more common since the 1990s. Long term drought and climate change both have influenced the frequency of dry years. Here we explore the probability of back-to-back low water years defined as both “Dry” and “Average Dry” based on the Upper Colorado River Endangered Fish Recovery Program’s definitions. Irrigation companies and water utilities likely have other definitions for “dry” years based on their water supplies. Nevertheless, flow conditions in the Colorado River are indicative of flow conditions elsewhere in the basin.

The following analysis of the probability of “Dry” and “Average Dry” years is based on the frequency of these water year types between 1991-2023.

## [1] "Given that the current year is a Dry Year, which has an annual probability of 31.4%, the probability that the next year will be Dry is 9.9% assuming the runoff volume is independent between years."

## [1] "Given that the current year is a Dry or an Average Dry Year, which has an annual probability of 60%, the probability that the next year will be Dry or Average Dry is 36% assuming the runoff volume is independent between years."

This frequency has increased over the period of record. We now evaluate the probability of back-to-back dry years based on the period of record.

## [1] "Back-to-back dry year have occurred 4 times over the period of record and 3 times since the year 2000."

The following figure considers how the probability of successive flow flow years has changed over time.

Autocorrelation

If a time series of data has a pattern or periodicity to it, this is referred to as serial or autocorrelation. The runoff volume in a given year may be correlated (or vary with) one or more previous years. That is, autocorrelation can exist between adjacent years or across multiple years (lagged autocorrelation). Below, the autocorrelation for April-July flow volume at Cameo is investigated for a lag of 1 to 10 years. Any correlation value above or below the blue dashed line indicates a statistically a significant correlation for that lag year’s value.

This analysis indicates a significant positive autocorrelation at a lag of one year and a significant negative autocorrelation at a lag of seven years. With a positive relationship between the runoff volume in a given year and the year prior, higher volume years are more likely to follow higher volume years and, conversely, lower volume years follow lower volume years. The seven-year lagged negative autocorrelation indicates that runoff volume tends to be higher if the volume was lower seven years prior, and lower if the runoff volume was higher seven years prior. This could indicate an approximately 14-year periodicity in runoff volume.

Recovery Program Flow Targets

The Upper Colorado River Endangered Fish Recovery Program (Recovery Program) has set flow targets for the 15-mile reach throughout the year. These include base flow and high flow targets, which are aimed at supporting basic habitat and survival needs for native fish during the low flow season as well as geomorphic and fish life cycle goals during the runoff season.

Peak Flow Targets

We begin by considering the magnitude and duration of high flows over the period of record. The following table outlines high flow targets and frequencies. These flow targets are published in the Programmatic Biological Opinion (USFWS, 1999) and came out of Osmundson et al. (1995). Each flow target is expected to be met a certain percentge of years. For example, the peak flow target for wet years is expected to be equalled or exceeded 25% of years.

Peak Flow Frequency Targets (Years of out 20)

Runoff Year Class	Target Flow Rate (cfs)	Relative Frequency (yrs out of 20)
Wet	>23,500	25%
Avg. Wet	21,750	50%
Avg. Dry	16,700	75%
Dry	12,900	100%

An overall negative trend is evident in the percent of years that each flow target has been met as well as the mean number of days above each flow target.

Another way to consider peak flow targets is by flow year type. As previously described, the flow years types (Wet, Avg. Wet, Avg Dry, Dry) are based on percentiles of period of record runoff volume between April and June.

The figure illustrates the number of days above a peak flow target associated with each water year type.

The figure above illustrates that peak flow targets are typically met in wet years (but not always). In the last several decades, peak flow targets met in other runoff year types.

Low Flow Targets

Low flow targets have also been set for the 15-mile reach based on the percentile of water year. The following plots show annual trends in number of years above low flow threshold targets as well as number of days per year during the low flow season (August to March). These flow targets have been established for the 15-mile reach below the Grand Valley Irrigation Canal Diversion. Therefore, the following analysis relies on the USGS gage below this diversion (Station ID 09106150).

Minimum Flow Targets by Percentage of Years Out of 20

Month	25%	50%	80%	100%
AUG	1,630	1,630	1,240	810
SEP	1,630	1,630	1,240	810
OCT	1,630	1,630	1,240	810
NOV	1,630	1,630	1,630	1,240
DEC	1,630	1,630	1,630	1,240
JAN	1,630	1,630	1,630	1,240
FEB	1,630	1,630	1,630	1,240
MAR	1,630	1,630	1,630	1,240

The following plots show trends related to flows falling below base flow targets for August to October.

Trends in base flow targets indicate an overall increase in the number of years and days below base flow targets for the late summer and early fall months (August to October). 15-Mile reach flows fall below the dry year base flow target of 810 cfs in most years (60% of years, 10-year average) with an average of 30 days below this target per year (10-year average).

Typical Hydrographs by Water Year Type

To visualize the magnitude, timing, and duration of annual hydrographs by water year type, we plot the daily flow interquartile range (shaded area) for all hydrographs under each category.

Based on recent examples of runoff years from each class, we now plot examples of three annual hydrographs from each class to demonstrate the flow seasonality.