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Western Plateau Planning Area Hydrology - Surface Water (Continued)

Click to view Figure 6.0-7

Figure 6.0-7 USGS Watersheds in the Western Plateau Planning Area

The U.S. Geological Survey (USGS) divides and subdivides the United States into successively smaller hydrologic units based on hydrologic features.  These units are classified into four levels. From largest to smallest these are: regions, subregions, accounting units and cataloging units.  A hydrologic unit code (HUC) consisting of two digits for each level in the system is used to identify any hydrologic area (Seaber et al., 1987).  A 6-digit code corresponds to accounting units, which are used by the USGS for designing and managing the National Water Data Network.  There are portions of three watersheds in the planning area at the accounting unit level: Upper Colorado River-Lake Powell; Little Colorado River; and Lower Colorado River, Lees Ferry to Lake Mead (Figure 6.0-7).

Lower Colorado River, Lees Ferry to Lake Mead Watershed

Most of the Western Plateau Planning Area is included in the Lower Colorado River, Lees Ferry to Lake Mead Watershed, which extends into the Upper Colorado River Planning Area.  The watershed is drained by the Colorado River, which flows southwest from Lake Powell to Lake Mead.  There are a number of perennial streams in the Kanab Plateau Basin that flow to the Colorado River including Kanab, Bright Angel, Nankoweap, Shinumo and Tapeats Creeks.  None of these streams have flow gages. In the Coconino Plateau Basin, major perennial tributaries are Havasu and Diamond creeks.  West of Diamond Creek, the only perennial flows are the Virgin River, which flows through the planning area from its headwaters in Utah to Lake Mead in Nevada, and an approximately one-mile reach of a tributary, Beaver Dam Wash.

Flow in the Colorado River downstream from Lake Powell is controlled by releases from Glen Canyon Dam, which has significantly impacted flow volumes and historic seasonal variations in flow as mentioned in the previous watershed discussion.  There are five streamflow gages along the Colorado River in this watershed. The three easternmost gages are located above the Little Colorado River and near Bright Angel Creek (see Figure 6.3-5). These gages have varying periods of record and show average annual flows of 8.5 to 11.2 maf a year. A gage with 79 years of record (Colorado River near Grand Canyon, Table 6.3-2), the only pre-dam gage, has the highest mean flow (11.2 maf) and highest maximum flow of 20.5 maf in 1984. The two westernmost gages are located near Havasu Creek and Diamond Creek (see Figure 6.1-5) and are post-dam gages. The only currently operating gage (above Diamond Creek) has a similar flow regime to the other post-dam gages in the watershed with a mean flow of 10.4 maf and a maximum flow of 15.97 maf (Table 6.1-2).

Prior to construction of the Glen Canyon Dam, flow in the Colorado was highly unpredictable with wide year-to-year variability and spring flooding.  Operation of the dam for electrical generation requires large water releases with daily and weekly fluctuations and releases during historically low flow seasons.  Provisions of the Record of Decision (1996) for the Glen Canyon Dam Final EIS and the Glen Canyon Dam Operating Criteria (1997) set restrictions on daily and hourly flows. The maximum flow may not exceed 25,000 cfs except for beach/habitat-building flows, habitat maintenance flows, or when necessary during above average hydrologic conditions.  Minimum flows are restricted to 5,000 to 8,000 cfs depending on the time of day.  Further, daily fluctuation limits are 5,000 cfs to 8,000 cfs depending on monthly release volumes. (USBOR, 2008)

A tree-ring-based reconstruction of over 500 years of Colorado River streamflow found as many as eight droughts similar in severity to the 2000-2004 drought period.  The reconstruction also suggests that the last 100-year period was wetter than the average for the last five centuries, and that average annual flows regularly vary from one decade to the next by more than 1.0 maf.  The most severe sustained drought (based on the lowest 20-year average) in the Upper Colorado River basin apparently occurred in the last part of the 16th century.  (Meko and others, 2007)

Colorado River

Colorado River through the Grand Canyon.  Prior to construction of the Glen Canyon Dam, flow in the Colorado was highly unpredictable with wide year-to-year variability and spring flooding.

The other major river in the watershed is the Virgin River, which drains an area of about 6,100 square miles. The river flows from its headwaters north of Zion National Park in Utah to Lake Mead.  Prior to construction of Hoover Dam it flowed to the Colorado River.  Now, its lower 20-30 mile former reach has been inundated by the Overton Arm of Lake Mead.  Dixon and Katzer (2002) estimated the Virgin River outflow to Lake Mead at 132,000 AFA.

The entire reach of the Virgin River within Arizona is perennial (AGFD, 1997). Reportedly, there were historic periods of no flow in the Virgin River above the Littlefield Springs (Figure 6.6-6), a collection of eight springs located over a distance of seven miles between the Narrows and Littlefield gages (see Figure 6.6-5 for gage location). These periods of no flow were determined from a gage installed upstream of the Littlefield Springs (1951-1956 and 1976) and were caused by irrigation diversions near St. George, Utah and seepage losses near Bloomington, Utah. (Trudeau and others, 1983)  Substantial seepage losses from the Virgin River to the groundwater system between the near St. George and Bloomington gages were reported by Trudeau (1979). This reach begins about a half mile north of the Arizona border and extends to St. George. However, post-1990 gage data and seepage measurements suggest that the historical seepage losses to the groundwater system in Utah are no longer occurring (Cole and Katzer, 2000). 

Virgin River near Littlefield

Virgin River near Littlefield.  Flow lost from the Virgin River to the groundwater system reenters the river via discharge from the Littlefield Springs.

In Arizona, seepage losses between 10 to 35 cfs were estimated upstream of the Narrows gage (Cole and Katzer, 2000). Flow lost from the Virgin River to the groundwater system reenters the river via discharge from the Littlefield Springs. Measuring discharge rates at the springs is difficult because they are located in the Virgin River channel and can only be observed during low flow when the sediment load is near zero (Dixon and Katzer, 2002).   An estimated 20 to almost 70 cfs (14,500 to 50,700 AFA) reenters the Virgin River via springs and groundwater discharge between the Narrows and Littlefield gages (Cole and Katzer, 2000). Since 1998 average annual flow in the Virgin River above the Narrows gage has been about 92,600 acre-feet.  Below the Narrows gage, average annual flow increases to 174,502 acre-feet at the Littlefield gage, with a 72 year period of record.

The short perennial reach of Beaver Dam Wash is supported by springs that collectively discharge over 1,100 gpm.  Beaver Dam Wash discharges to the Virgin River north of the Littlefield gage.

A number of major springs issue from the Redwall and Muav limestones and to a lesser extent, the Tapeats Sandstone, in the vicinity of the Colorado River in the Kanab Plateau and Coconino Plateau basins. The largest are Havasu Springs in the Coconino Plateau Basin with a discharge of about 28,500 gpm, and Tapeats Spring in the Kanab Plateau Basin with a discharge of about 18,700 gpm.  Havasu Creek is perennial below Havasu Spring, located upstream of the village of Supai, and contains moderate levels of calcium, magnesium and bicarbonate from the springs.  Calcium carbonate precipitates out of the spring water, forming travertine deposits along the creek bottom/bed. 

A major flash flood event occurred on Havasu and Cataract Creeks from August 15th- 17th, 2008, causing severe damage to Supai Village and nearby campgrounds on the Havasupai Reservation and stranding tourists and residents. Estimated flood flows were 6-7000 cfs.  In response, two streamflow and precipitation gages were installed upstream to provide timely and accurate flood warnings to the Havasupai Nation and campgrounds along Havasu Creek.

Roaring Springs, located 3,000 feet below the North Rim, emanates from a cave in the Muav Limestone above the intersection of the Roaring Springs and Bright Angel faults. It has a discharge of almost 2,000 gpm and is the water supply for the North and South Rims of Grand Canyon National Park (USBOR, 2002).

Havasu Falls

Havasu Falls in 2007.  A major flash flood event occurred on Havasu and Cataract Creeks from August15th- 17th, 2008, causing severe damage to Supai Village and nearby campgrounds on the Havasupai Reservation and stranding tourists and residents.

A group of major springs with discharge rates between 11 and 90 gpm are found in the vicinity of Moccasin and Kaibab in the north-central part of the Kanab Plateau Basin.  Studies at Pipe Spring National Monument indicate that spring discharge is from a sandstone unit of the Kayenta Formation.  Fine-grained sediments below the unit create a confining layer that restricts vertical water movement and forces groundwater to move along bedding planes and fractures in the Navajo Sandstone and the upper unit of the Kayenta Formation.  In the monument, discharge at Pipe Spring declined between 1976 and 2003 but increased at Tunnel Spring for reasons that are unclear.  The combined spring discharge declined about 0.5 gpm per year between 1986 and 2001 (Truini and others, 2004). 

A handful of major springs are found in the other basins in the watershed.  In the Grand Wash Basin, three major springs, (Tassi, Whiskey and an unnamed spring) discharge from the basin-fill aquifer where it overlies a confining unit, the Muddy Creek Formation (Bales and Laney, 1992).  This may be the case with other springs in the basin.  The only major spring in the Shivwits Plateau Basin, with a measured discharge of 331 gpm is found at the mouth of Spring Canyon at the Colorado River.

There are two impaired stream reaches in the watershed. Twenty-eight miles of the Colorado River from Parashant Canyon to Diamond Creek are impaired due to selenium and suspended sediment concentrations (Table 6.1-7). These same constituents are responsible for the impairment designation of ten miles of the Virgin River from Beaver Dam Wash to Big Bend Wash (Figure 6.6-10).

 

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