7371 - Laserfiche WebLink

Home Browse Search

Once the sla~c-dJschar!!c relationship and the flow in thc- side channel arc knl1wl1. the rest of thc- anahsl~ is compara- tively simple The dls("har~es to he slIllulatcd In the side channel arc determined from the empirical relation hctween main channel and side channel discharges (Fig. )), The amount of microhabitat calculated for each of these dis- charges is weighted according to reach length. and added to similarly weighted values for the main channel at cor- responding discharges. On the basis of predicted main chan- nel water surface elevations and the inlet elevation at transect H, we detennined that flow in the side channel ceases when the main channel discharge is less than 100 m3.s-1 Therefore. when one computes total reach habitat for lower discharges, zero flow would be simulated in the side channel. The foregoing discussion concentrates on procedures for simulating low flows, but most microhabitat analyses also require attention to high flows. In a divided flow section it is necessary to survey the bed cross section to the highes; point on the island, to detennine the flow at which the island becomes inundated. When that discharge is reached, the divided flow model no longer holds, and must be replaced by a single-channel model. Whereas different numbers of transects on either side of the island could be used for the divided flow model, the single-channel model requires that each transect be completed across the entire channel. At the Palisade site, the single channel model would require the extension of transects 6R through D, and the replacement of transects E through H. However, the characteristics of the main channel between transects 5 and 7 were highly uni- form. The extension of the side channel cross sections could be performed with little more than a bed profile (which could be measured at any discharge) and a water surface profile that was measured at any discharge when the island was submerged. This combination is fortunate because hydrographic survey measurements under potentially dan- gerous high flow conditions are not required. The only criti- cal high flow measurement is the water surface elevation which can be taken near shore. ' T~b~tary B.ackwat~rs - Although the analysis of many speCialized microhabitat types can be similar to that used at the Palisade site, one type requires a different treatment. The mouths of tributaries are important microhabitat areas in some str~ms, and often function as refuges during ~xtremely high or low flows. Treating these areas as mdependent stream reaches would seem to be a logical approach, e~cept that they are not independent of the con- fluent stream. The stage in the tributary is often a function ~fth~ stage in the confluent stream. but the velocity distribu- tion Is.more closely related to the discharge in the tributary. OccaSIOnally the stage is not influenced by the confluent stream,. but de~ermined primarily by tributary. discharge. AnalYSIS of thiS type of variable backwater requires the development of a stage-discharge relation in the confluent stream, s~ that the backwater elevation in the tributary can be detennmed. A series of simulations is then conducted for the same tributary discharge, but with different starting water surface elevations corresponding to the discharge in the ~onfluent stream. This procedure results in a family of habitat values, each depending on the discharge in both streams. Habitat D~namics A fe<lture of rivennc- c-nvinlllmenh is thl" seasonal. monthly. daily. and even hourly changes in the microhabitat quality and quantity. The most olwious change is complete dewatering, Beyond simple dewatering. the usability of the water column for a particular aquatic organism is very much a function of the temperature. fluid movement and hydrau- lics as expressed by point velocities and depths. T~cking of the quality of these microhabitats over time becomes a very important consideration in riverine fishery manage- ment. Innuence of River Hydrology Stream flows in rivers vary from year to year and from day to day. The variation in habitat quality of a given river is related to the general climate pattern, the variation of run- off, and the nature of groundwater storage in the basin. The typical approach to describing riverine hydrology is through the use of stream discharge monitoring at various points within a river basin. At these monitoring points, com- monly referred to as "stream gaging stations," a river stage versus discharge relation is developed. Continuous records of stage are automatically recorded and later transformed into discharge values over the monitoring time period. For riverine fishery habitats, historical flow records over at least a 100yr period are needed. Various hydrologic techniques are used to extend records from one part of a river basin to another and to synthesize records and fill data gaps. Each river segment that has a unique flow pattern, there- fore, needs a series of daily or monthly flow values that can be defined as a hydrologic baseline. Water management and development activities simulated with the aid of reservoir operating and water routing models can be used to create alternative hydrologic time series, for various water man- agement options over this same historical "baseline" time period. These measured and synthesized hydrologic time series are typically summarized and presented as flow dura- tion curves with daily, monthly, seasonal or annual time periods. Figure 6 is an example of a flow duration curve for 1200 300 ::- I OJ ,; oS w 0 a:: < :I: 30 0 ~ c DURATION PLOT OF DAILY DATA SEPTEMBER 1951-1984 COLORADO RIVER NEAR COLORADO-UTAH STATE LINE DRAINAGE AREA - 4.621.337 Hectares 3 0,13 1 5 20 40 80 80 95 99 99,87 PERCENT OF TIME INDICATED VALUE WAS EOUALED OR EXCEEDED FIG. 6. Flow duration curve for month of September, stateline gage on Colorado River. 21 ....