Hydraulic Modeling of Ozarks Stream Habitats
This section includes supplemental information about selected topics in the main document text.
A.1. The Ozarks Stream Geomorphology Project:
The goal of the Ozarks Stream Geomorphology Project (OSGP) has been to increase understanding of links between environmental stresses and stream community dynamics in this biologically diverse area of the mid-continent. The project is interdisciplinary and has involved hydrologists and biologists from the Water Resources Division, Biological Resources Division, and Missouri Cooperative Research Unit of the U.S. Geological Survey (USGS). Research has focused on understanding factors that control stream morphology in the Ozarks, the role of stream processes in determining aquatic habitat, and the influence of physical habitat characteristics on aquatic community structure (Figure A1.1). By working at a range of scales, from basin-wide to detailed study reaches, the various components of the project have sought to isolate variables and determine the sensitivity of aquatic communities to future environmental stresses.
A.2. Surveying and biological sampling:
As part of the Ozarks Stream Geomorphology project, the USGS has established a long-term physical-biological monitoring network on the Buffalo and Jacks Fork Rivers (Figures 1.2 and 1.3). At three reaches on each river, a network of cross sections has been resurveyed annually to document year-to-year channel dynamics at a scale applicable to aquatic communities (McKenney and Jacobson, 1996). This data provides a volumetric measure of channel changes and channel response to physiographic and flood-event controls. In addition, it provides detailed physical habitat information that has been used in the design of intensive biological sampling projects. Biologists at the USGS Cooperative Research Unit (University of Missouri-Columbia) have carried out multiple studies to evaluate the links between physical habitat characteristics and fish and invertebrate populations (Peterson and Rabeni, in prep; Doisy and Rabeni, in review). The modeling presented in this document is based on the detailed survey data and biological data collected at the Ratcliff Ford study reach on the Jacks Fork River (Figure 1.4).
A.3. Derivation of the regression relationship between flows at Eminence and Ratcliff Ford:
The USGS has maintained three gages in the Jacks Fork basin over different time intervals since 1921 (Table 1.1). The longest record is for the gage at Eminence, which has recorded stream flow continuously since 1921. Between 1965 and 1980, the USGS also maintained a gage to record flow from Alley Spring the largest spring in the Jacks Fork basin (Figure 1.2). During this time period, Alley Spring had an average daily flow of 4 cms and contributed 30% of the lower Jacks Forks daily mean flow (as recorded ten kilometers downstream at Eminence). In 1993, the USGS began gaging flow of the Jacks Fork just upstream of Alley Spring (this gage is referred to as the Jacks Fork at Alley Spring).
The modeling reach at Ratcliff Ford is not continuously gaged, therefore, flow duration characteristics must be estimated from the record of gages downstream. However, establishing relationships between upper basin discharges and those recorded at the stream gage at Eminence is complicated by the presence of Alley Spring. Limited dye tracing showed Alley Springs watershed area to extend beyond the Jacks Fork basin (Vineyard and Feder, 1974). This suggest that Jacks Fork discharges may not scale directly with watershed arealocations downstream of the spring receive discharge from a disproportionately large and unknown watershed area. Gage records also suggest that the importance of the spring changes with dischargeduring storm events, overland flow dominates, the karst system is bypassed, and the relative proportion of spring flow diminishes.
We analyzed stream gage records from all three gages to evaluate the affect of Alley Spring discharge on regression relationships between the Eminence gage and sites above the spring. Comparison of these three gages is complicated by the lack of overlap between records, all three gages have never run simultaneously. Therefore, we used a regression relationship between Eminence and the gage on the Jacks Fork at Alley Spring (data from 1992-1996) to estimate flows above Alley Spring during the period when Alley Spring itself was gaged (1965-1980).
The graph of flow duration (Figure A3.1) for 1965 to 1980 for all three gaging sites indicates that discharge of the spring rises in tandem with discharge of the Jacks Fork. This indicates that spring flow is not disconnected from surface water hydrologysimilar to flow in the main channel, spring flow responds in the short term to storm events. However, the proportion of the flow at Eminence contributed by Alley Spring decreases with discharge. The percentage of flow at Eminence contributed by Alley Spring decreases from 64% for flows exceeded 99% of the time to 14% for flows exceeded 1% of the time. This indicates that run-off during a storm increases at a faster rate than does flow from the karst system and the karst system is bypassed for less frequent, more intense storms. However, the data for flows exceeded 1% of the time, shows that the spring flow from Alley Spring is still an important contributor to the discharge at Eminence even for infrequent storm events.
This conclusion suggested that we might obtain a better regression relationship between the discharge at Eminence and at Ratcliff Ford if we could separate out flow from Alley Spring. To do this, we found discharges at Eminence in the 1965-1980 gage record that were equivalent to discharges recorded at Eminence and were on days for which discharge had been measured at Ratcliff Ford (between 1992 and 1996). We then averaged spring flow values recorded on the same days as these Eminence discharges to obtain an estimate of spring flow for a given Eminence discharge. Regressions of discharges at Ratcliff Ford to discharges at Eminence (including and excluding spring flow) show that the regression relationship is not improved by subtracting out spring flow. Storm discharges at Eminence scale with storm discharges at Ratcliff Ford despite any buffering effect of discharge from Alley Spring. This observation indicates that the presence of Alley Spring does not invalidate the regression relationship between discharges recorded at Ratcliff Ford and at Eminence.
A.4. Derivation of the stage/discharge relationship for Ratcliff Ford
We used the slope-area method and the program XSPRO to estimate discharge for peak flows recorded by a crest-stage gage at the Ratcliff Ford study site. This method is based on the Manning equation and calculates discharge from water-surface slope, cross-sectional area, and an estimate of channel roughness. We estimated water-surface slope (0.18%) for these high discharges from high water marks that were surveyed and correlated with crest-stage gage measurements following peak flow events. Initially, we obtained roughness estimates within XSPRO from resistance equations derived by Thorne and Zevenbergen (1982) and Jarrett (1990). Because these equations predict that roughness will decrease as stage increases they led to extremely high discharge estimates (in some cases approaching those recorded for the same event on the Jacks Fork at Alley Spring). Banks at Ratcliff Ford are heavily vegetated and when they are submerged during overbank flows, it is likely that roughness is higher than that estimated by XSPRO. We adjusted roughness to compensate for vegetation and obtained lower discharge estimates. Roughness values at flood stage ranged between 0.04 and 0.055, values consistent with those given for flood stage in a pool-riffle stream by Chow (1959). Analysis in the two-dimensional flow model, SMS, also supports these discharge estimates since the discharge/stage pairs produce similar water surface slopes to those recorded by high water marks.