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                                   S.W. Wolff
                                  T.A. Wesche

                   Symposium Proceedings         WWRC-88-29


                Proceedings of the Water and the West Symposium
                               Wyoming Division
                      American Society of Civil Engineers

                                  S.W. Wolff
                     Department of Zoology and Physiology
                         Wyoming Water Research Center
                             University of Wyoming
                                Laramie, Wyoming

                                  T.A. Wesche
                         Wyoming Water Research Center
                             University of Wyoming
                                Laramie, Wyoming

                                  April, 1988


                       Steven W.Wolff and Thomas A. Wesche
                           Wyoming Water Research Center
                               University of Wyoming
                                 Laramie, Wyoming


One aspect of instream flow regimes now being actively debated is the need for channel maintenance or flushing flows. However, little quantitative information exists as to how different types of channels, particularly higher elevation, mountain stream systems, respond to changes in the flow regime resulting from water development activities. Our study is attempting to begin to answer some of these questions. This paper discusses part of that study and addresses the effect flow diversion has had on higher elevation stream systems located In Wyoming and Colorado.


Over the past 20 years, the maintenance of suitable instream flows below water development projects In the western United States has been recognized as environ- mentally desirable and a cost developers, in many cases, must be willing to incur. Currently, one aspect of instream flows which is being actively debated by water developers and natural resource management agencies is the need for, and the determination of, flushing and channel maintenance flow requirements. Such instream flows simulate the natural spring runoff hydrograph and are felt to be necessary to maintain conveyance capacity of stream channels by reducing aggradation and encroachment of riparian vegetation, and to remove accumulated fine sediments from critical fish habitats.

Given the quantities of project water typically required for flushlng/ channel maintenance purposes and the associated costs of that water, basic questions are being raised regarding the quantitative response of stream chan- nels to flow regulation. Should certain channel types respond more slowly to flow regulation, the argument can be made that the magnitude and duration of some flushing regimes can be reduced while still maintaining conveyance capacity and aquatic habitat quality.

In 1986, the Wyoming Water Research Center began a project to investigate the quantitative response of higher elevation stream channels in the central Rocky Mountains to flow depletion or augmentation resulting from water develop- ment. This project Is not scheduled to be completed until late 1988, though this paper summarizes part of the project (diversions on mountain streams) and discusses some results to date.


Work began in July of 1986 with the determination of potential sites. Selection of a particular stream for actual sampling was done onsite. Field sampling of sites was done in the summer and fall of 1986 and 1987, and con- sisted of sampling stream reaches immediately above and below a diversion structure. Data collected at each reach included mean channel width and depth, stream gradient, composition of the riparian zone, and composition of the streambed and banks. Several photographs (black and white prints, and color slides) were taken at each site as well. All study reaches were located in the first stable, straight reach above/below the diversion structure which occurred out of the area of construction impact.

Based on the field data, conveyance capacity using mean channel width and depth, and channel slope-was calculated, for each site. Hydrologic, and drainage basin data are currently being gathered and analyzed for all study reaches. Channel stability of study reaches is also being assessed using the Stream Reach Inventory/Channel Stability Evaluation (Pfankuch 1975).


As mentioned earlier in the paper, analysis of the channel response data collected on mountain streams is not yet completed. We anticipate a project completion report will be available late in 1988. Therefore, the results presented here should be considered as preliminary and as such, will be restricted to general data trends.

Field measurements of channel width and depth were made at 39 study sites on 19 streams in northern Colorado and southern Wyoming. Site elevations ranged from approximately 7,400 to 9,800 ft above mean sea level, while surveyed water surface slopes varied from less than 1.0 up to 9.8 percent. The diversion structures on the study streams ranged in age from over 100 years down to less than 25 years and depleted streamflow by 5 to almost 100 percent of average annual water yield. As many of the study streams are ungaged, synthesis of discharge records is now underway. Applying Rosgen's (1985) channel typing system, 11 of the 39 sites were classified as A channels, 14 as B channels, and 14 as C.

A comparison of channel characteristics above and below the diversion struc- ture on each stream is presented in Table 1. Response variables considered to date in our analysis include channel width, channel depth, the ratio of width to depth, cross-sectional area and channel conveyance capacity. The response of these parameters to flow depletion has been highly variable. Conveyance capacity has shown the greatest variability, ranging from a reduction of 85 percent below the diversion on North Brush Creek to an Increase of 101 percent below the Fool Creek structure. Channel width was the most constant of the variables, showing a 40 percent reduction at a low gradient site below the North Fork of the Little Snake River diversion and a 24 percent widening on Fool Creek. Cross-sectional area, depth and the ratio of width to depth were intermediate in response.

The general trends of the data from Table 1 are presented in Table 2. As shown, channel shrinkage was found to occur below approximately 50 percent of the diversion structures. This phenomena was not observed at the remaining half of the study streams. It is apparent that additional analysis, taking into consideration such factors as channel slope, sediment yield, elevation, vegetation, and magnitude and duration of streamflow depletion, is needed to begin to explain the observed responses. This effort is now well underway.


While data analysis is not yet complete and any conclusions drawn at this time must be considered preliminary, it is quite apparent that the physical response of mountain stream channels to flow depletion is highly variable. Certain of our study streams were reduced in size due to the processes of vegetative encroachment and channel aggradation, while others exhibited no such loss of conveyance capacity. Further analysis is needed to explain this variation.

The channel maintenance issue is a complex one. Before instream flow regimes are prescribed below water development projects to preserve the channel capacity and competence, it would appear that consideration should be given to the type of stream channel involved, the sediment loadings to the system, the transporting

TABLE 1.  Summary of channel response to flow depletion for mountain streams.*

      SITE                  WIDTH      DEPTH       AREA         W/D       C.C.

  Above Wolfard Canal       25.00       2.00       50.00       12.50      236.86
  Below Wolfard Canal       26.20       2.00       52.40       13.10      364.11

  Above Pilson Ditches      19.80       2.50       49.50        7.92      421.07
  Below Pilson Ditches      21.30       1.50       31.95       14.20      180.72

  Above Diversion           10.10       1.00       10.10       10.10       36.04
  Below Diversion (steep)   10.50       1.00       10.50       10.50       58.97
  Below Diversion (flat)     6.10       1.00        6.10        6.10       22.77

  Above Supply Canal        27.90       2.00       55.80       13.95      601.78
  Below Supply Canal        30.40       2.00       60.80       15.20      655.93

  Above Highline Ditch      29.80       2.00       59.60       14.90      291.36
  Below Highline Ditch      19.50       1.50       29.25       13.00       44.42

  Above Vasquez Diversion   26.40       1.88       49.63       14.04      335.45
  Below Vasquez Diversion   17.60       1.30       22.88       13.54       81.80

  Above Diversion           17.60       1.50       26.40       11.73      167.42
  Below Diversion           18.10       1.29       23.35       14.03      102.01

  Above Diversion            5.00       0.82        4.10        6.10       17.82
  Below Diversion            6.20       0.85        5.27        7.29       35.78

  Above Diversion            7.60       1.87       14.21        4.06      177.58
  Below Diversion            8.20       1.15        9.43        7.13       78.15

  Above Diversion           19.20       1.34       25.73       14.33      154.01
  Below Diversion           21.60       1.43       30.89       15.10      193.95

  Above Diversion            7.30       0.86        6.28        8.49       36.81
  Below Diversion            5.80       0.85        4.93        6.82       21.20

  Above Diversion            2.20       0.83        1.83        2.65       10.07
  Below Diversion            2.00       0.71        1.42        2.82        6.17

TABLE 1 (cont'd).  Summary of channel response to flow depletion for mountain 

      SITE                  WIDTH      DEPTH       AREA        W/D          C.C.

  Above Diversion           16.10       1.13       18.19       14.25       87.53
  Below Diversion           11.90       1.26       14.99        9.44       82.66

  Above Diversion           10.00       0.93        9.39       10.86       28.38
  Below Diversion            9.00       0.84        7.56       10.71       54.02

  Above Diversion           15.70       1.21       19.00       12.98      195.24
  Below Diversion           13.80       1.99       27.46        6.93      312.04

  Above Diversion            9.70       1.35       13.10        7.19      110.92
  Below Diversion            9.40       1.53       14.38        6.14      142.60

  Above Diversion           11.00       1.57       17.27        7.01      262.45
  Below Diversion           10.00       1.57       15.70        6.37      200.11

  Above Homestake Tunnel    21.00       1.50       31.50       14.00      134.26
  Below Homestake Tunnel    22.80       1.80       41.04       12.67      195.92

  Above Diversion           14.10       1.21       17.06       11.65      142.37
  Below Diversion           13.50       1.27       17.15       10.63      166.76
*WIDTH = Mean channel width (feet)
 DEPTH = Mean channel depth (feet)
 AREA  = Cross-sectional area of channel (square feet)
 W/D   = Width-Depth ratio
 C.C   = Conveyance capacity (cubic feet per second)
capability of the flow regime in relation to these loadings, and the factors which govern the establishment and growth of streamside vegetation. We hope that when completed, the results of this study will help to provide some of the insight needed.
TABLE 2.  Trends in channel response of twenty mountain streams in Wyoming and
          Colorado to flow depletion.*

                                     RESPONSE VARIABLE (Number of Streams)_________

CHANNEL RESPONSE**            WIDTH       DEPTH        W/D        AREA        C.C.

       +                        9            7           9           9         10

       -                       11            8          11          11         10

       0                        0            5           0           0          0

    TOTAL                      20           20          20          20         20
 *WIDTH = Mean channel width (feet)
  DEPTH = Mean channel depth (feet)
  AREA  = Cross-sectional area of channel (square feet)
  W/D   = Width-Depth ratio
  C.C   = Conveyance capacity (cubic feet per second)

** + indicates variable increased below diversion
   - indicates variable decreased below diversion
   0 indicates no difference in variable above and below diversion   


Pfankuch, D.J. 1975. Stream Reach Inventory and Channel Stability Evaluation. USDA Forest Service, Missoula, Montana.

Rosgen, D. 1985. A stream classification system. In Riparian Ecosystems and Their Management: Reconciling Conflicting Uses. GTR/RM-120, USDA Forest Service, pages 91-95.

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