Man's Influence on Freshwater Ecosystems and Water Use (Proceedings of a Boulder Symposium, July 1995). IAHS Publ. no. 230, 1995. 275

Changes in transport capacity in the lower Gunnison , Colorado, USA

ROBERT T. MILHOUS National Biological Service, 4512 McMurry Ave., Fort Collins, Colorado 80525, USA

Abstract The Gunnison River is a southwestern Colorado (USA) river with a mean of 2290 hm3 year"1. The lower reach of the river is habitat for the Colorado Squawfish (Ptychocheilus lucius), an en­ dangered species. The discharge of sediment and water in the river has been modified by construction of major with a combined capa­ city of 1475 hm3, and by diversions for irrigation of about 94 300 ha and for municipal and industrial purposes. The substrate of the lower river is gravel and cobble, with significant quantities of sand and fines both on the surface of the cobbles and boulders and within the bed material. The dso size of the armour layer is 94.5 mm, of the substrate below the armour 52.0 mm and of a fine material on the surface of the armour in some locations 0.055 mm. The fine material is an important aspect of the habitat for aquatic animals. Hydraulic modelling of a 2 km reach indi­ cates a discharge of 300 m3 s"1 is required to flush fines from the system and improve the habitat for aquatic animals. The ability of the river to flush fines and sand has been much reduced by the water control works.

INTRODUCTION

The Gunnison River, located in Southwestern Colorado (USA), has a mean discharge of 73 m3 s*1 and a 75 m width just upstream of its junction with the . The lower reach of the river is habitat for Colorado River Squawfish {Ptychocheilus lucius), an endangered species. The flows of sediment and water in the river have been modified by the construction of reservoirs and by major diversions for irrigation and for municipal and industrial purposes. This paper investigates the impact of the changes in water management on the ability of the river to transport sediment.

Gunnison River and basin

The upper (eastern) part of the basin is in the Rocky Mountain physiographic province and the rest in the Colorado Plateau province. Streamflow and sediment data measured at the US Geological Survey Whitewater gauging station are used for most of the analysis described in this paper. The drainage area of the basin above Whitewater is 20 534 km2. Downstream of Whitewater the river enters the Grand before joining the Colorado River in Grand Junction. About 94 300 ha are irrigated in the basin above Whitewater. 276 Robert T. Milhous

The use of water for irrigation and the construction of storage reservoirs in the Gunnison River basin have had a major impact on the discharge of water and sediment in the river. Many small reservoirs and diversion works exist in the basin. The first major water control work constructed was the Gunnison Tunnel, which diverted water from the upper Gunnison River for use in the Umcompahgre basin (a sub-basin in the Gunnison basin). Diversions through the tunnel started in 1910. There was a significant increase in major storage capacity in the Gunnison River basin between 1917 and 1993. For analytical purposes the water storage history of the basin can be divided into three periods. The period from the beginning of discharge records (in 1896 but with missing years before 1917) until the completion of Taylor Park (137 hm3) in 1937, the period following construction of Taylor ParkReservoir until completion of Blue Mesa Reservoir (1020 hm3) in 1965 (in 1966 there was 1187 hm3 of major reservoir capacity) and the period from 1965 to 1993 (reservoir capacity increased to 1475 hm3). The following periods were used to compare the impacts of the reservoirs and water uses: (a) water years 1917-1936, (b) 1940-1965 and (c) 1968-1993. The years immediately following a period were not used in order to account for the filling period for the reservoirs. The reservoirs in the upper Gunnison basin and the use of water for irrigation have had a significant impact on the daily streamflows in the lower river but less impact on the annual flows. Mean annual streamflows and average maximum daily discharges are given in Table 1. The various uses of water have reduced annual flows by roughly 20 m3 s"1 at the present time. The early 1980s were very wet compared with previous periods, which is the reason the 1968-1993 period has a higher mean flow than the 1940- 1965 period. The lower Gunnison River can be characterized as a cobble and gravel river, with considerable sand and fine sediment on the surface of and among the cobbles and gravel. At a study site (Dominguez Flats) in the lower Gunnison River about 61 km up­ of the junction with the Colorado River and about 38 km upstream of the Whitewater gauge, the bankfull width of the Gunnison River averages about 75 m and the bankfull discharge is about 500 m3 s"1. The substrate is heterogeneous with three components: an armour layer on the bed surface, the substrate material just below the armour and sand and fines in and around, and sometimes over, the armour at low to moderate flows (ephemeral material). The armour is one grain thick and is generally much larger than the other two components. The substrate usually has all of the sizes found in the armour and in the ephemeral material, as well as the sizes in between. Each component must be sampled individually to characterize the bed material. The bed

Table 1 Impact of changes in water management on the mean annual and peak daily discharges of the Gunnison River near Grand Junction, Colorado.

Period Mean annual discharge Peak daily discharge Years in period (m3 s"1) (m3 s"1) 1917-1936 80.3 488 20 1940-1965 67.7 395 26 1968-1993 74.4 261 26 Changes in capacity in the lower Gunnison River, USA 277

material of a cross- in the Dominguez Flats reach had a median size of the armour layer of 94.5 mm, of the substrate 52 mm and of the ephemeral material 0.055 mm. The has been sampled at the Whitewater gauge. The suspended load as related to the discharge is given in Figure 1. The equation for the line shown on the diagram is QSL = 0.24 * Q2, where QSL is the measured suspended sediment load (t day"1) and Q is the discharge (m3 s"1).

Sediment and squawfish habitat

The Colorado squawfish is an endangered fish native to the warm water reaches of the Colorado River. The Colorado squawfish is a predator, the adults feed on other fish. Young fish feed on small invertebrate animals, but as they grow they become increa­ singly dependent on fish (Behnke & Benson, 1980). Colorado squawfish spawning behaviour has two phases (Tyus et al., 1987): (a) a resting phase in pools or large shoreline eddies where the fish rest and feed between spawning forays or where males gather around females until they are ready to deposit eggs; and (b) a -fertilization phase on cobble bars where the females deposit adhesive eggs on and among the cobbles and males fertilize them. Spawning occurs between late June and mid August. Fine sediment and sand covering cobble bars or filling the spaces among the cobbles can have three impacts on the squawfish: make the cobble bars unusable for spawning; reduce the population of invertebrate animals living on and among the cobbles; and reduce the populations of small fish using the cobbles as habitat. Periodic flushing flows are needed to remove the sand and fines. Flushing flows are probably not needed every year and the interval between flushing flows is related to the quantity of sediment transported by the river.

SEDIMENT TRANSPORT CAPACITY

The use of a sediment transport capacity index to investigate changes in the sedimenta­ tion process in a river was presented previously by Milhous (1992). As used in this paper, a sediment transport capacity index for a day is calculated from the daily discharge and summed over the number of days in a period to give a sediment transport index for the period. The equation for the sediment transport index is:

STCI = E[(fi - QCRT)/QREF)b

where STCI is the sediment transport capacity index, Q is the daily discharge, QCRT is a critical discharge and QREF is a reference discharge. The power term, b, is usually the power term obtained from a sediment load vs discharge equation. In this paper the power term is 2.0. The critical discharge is not fixed but varies depending on the logic on which the index is based. Four logics are used here. Robert T. Milhous

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Fig. 1 Total suspended load as related to discharge of the Gunnison River near Grand Junction. Data from the WATSTOR database of the US Geological Survey and the files of the US Bureau of Reclamation.

Logic 1: Critical discharge required to clean the of fines and sand

The method used to calculate the critical discharge required to flush fines from the stream bed is given in Milhous et al. (1994). The required bed cleansing discharge was found to be 300 m3 s"1. A one-dimensional model was used.

Logic 2: Discharge needed to cleanse fines and sand from the pools

The bed shear stress in the pools is higher than in the cross-channel bars and if the discharge exceeds some critical discharge. The assumption is that pools are cleansed of fines and sand if the bed shear stress in the pools is larger than on the bars and riffles. A one-dimensional model was used to determine that the critical discharge (equal shear stress in a pool and ) is 145 m3 s"1 for one part of the study reach.

Logic 3: Minimum streamflow for effective transport of sand

The stream must transport sand to prevent the bed from being covered with sand and effectively converting the gravel/cobble bed to a sand bed. The critical discharge required for effective sand transport was obtained empirically from the measured sand vs streamflow graph. The critical discharge for sand transport is 85 m3 s'1.

Logic 4: All discharges are important in the sediment transport process

For this logic the critical discharge is zero. By comparing the sediment transport Changes in sediment transport capacity in the lower Gunnison River, USA 279 capacity index calculated with a critical discharge of zero to one based on one of the other three logics, some idea of the impact of changes in the system on the ability of the stream to move fines can be obtained.

CHANGES IN SEDIMENT TRANSPORT CAPACITY INDEX

The four logics were used to determine the differences in sediment transport capacity in the three periods defined by the construction of major storage reservoirs. The results are given in Table 2. The cleansing power of the river is much reduced, but the ability of the river to move the finer material downstream has not been reduced as much. The transport capacity with critical discharge of zero has actually increased between August and February. Also, the percent of the total capacity available between August and February has increased from 4-5% to 21%, which indicates the amount of fines lying on the stream bed during the winter months has probably been increased by the construction of the reservoirs. This factor could be important in limiting production of the invertebrates needed as food by the young Colorado squawfish hatched the previous summer. The capacity in June for 1968-1993 is 32% of the 1917-1936 capacity, a 68% reduc­ tion. In contrast the capacity change for the whole year was a reduction of 49%. Colorado squawfish spawn in late June through mid August. The reduction of total sediment capacity in June means the spawning areas could have more fine sediment among the cobbles and in the voids now than during the 1917-1936 period. Duration curves for the sediment transport capacity index for the case of cleaning the stream bed (critical discharge of 300 m3 s"1) are shown for the three periods in Fig. 2. The frequency of bed cleaning has been much reduced. This reduction has impacted the quality of the habitat because the bed accumulates fines and sand without being flushed for much longer periods. At this point the knowledge base is not adequate to support a change in water management in the basin to improve the habitat for Colorado squawfish. The changes in sediment transport capacity do show the dynamics of sediment transport have changed. The dynamics of the interchange of sediment between the water column and the stream bed must be understood before changes in water management are instituted.

Table 2 Impact of changes in water management on the sediment transport capacity index for the Gunnison River near Grand Junction, Colorado.

Period Critical discharge (m3 s"1) 300 145 85 0 0 0 Annual Annual Annual Annual Aug.-Feb. June 1917-1936 31.8 108.1 166.7 308.8 13.57 125.8 1940-1965 14.8 63.6 105.5 216.3 10.49 81.8 1968-1993 4.6 25.7 49.1 158.2 32.74 40.7

Reference discharge of 150 m3 s"1. Robert T. Milhous

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°Û°000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 Percent equal or exceeded •*- 1899-1936 -o- 1940-1965 -*- 1968-1993 Fig. 2 Duration curves of the sediment transport index for three periods. The period 1889-1936 includes the years 1889,1902-1906 and 1917-1936. Each period consists of 26 years. Critical discharge is 300 m3 s"1 and the reference discharge 150 m3 s"1.

Acknowledgements This paper reports on work under way as part of a joint project between the US Bureau of Reclamation and the National Biological Service to develop flushing methodologies. Russ Dodge and Perry Johnson of the USBR Hydraulics Branch have contributed much to the project and to the work leading to this paper.

REFERENCES

Behnke, R. J. & Benson, D. E. (1980) Endangered and Threatened Fishes of the Upper Colorado River Basin. Colorado State University, Fort Collins, Colorado. Milhous, R. T. (1992) Water and sediment in the middle Rio Grande valley, New Mexico. Use of a sediment transport capacity index. In: Proc. 12lhAGU Hydrology Days (ed. by H. J. Morel-Seytoux). Hydrology Days Publications, 57 Selby Lane, Atherton, California, USA. Milhous, R. T., Dodge, R. A. & Johnson, P. L. (1994) Bed material and numerical modelling in a gravel/cobblebed stream. In: Hydraulic Engineering '94 (ed. by G. V. Cotroneo & R. R Rumer), 1055-1059. ASCE, New York. Tyus, H. M., Jones, R. L. &Trinca, L. A. (1987) Green River Rare and Endangered Fish Studies, 1982-1985. US Fish and Wildlife Service, Vernal, Utah.