The average annual precipitation in the Missouri portion of the watershed ranges from 40 to 44 inches. This is the second highest region of precipitation in the state, lead only by the Southeast Lowlands. The average annual watershed rainfall ranges from 12 to 16 inches, and average annual evaporation is 55 to 60 inches. Most rainfall occurs during the months of March, April, and May, and the driest period is December, January, and February. The average maximum rainfall for a 24-hour period is 2.5 to 3 inches expected every two years and 5 to 6 inches for a 24-hour period expected once every 25 years (MDNR 1986a).
The United States Geological Survey (USGS) has maintained gaging and water quality stations throughout the watershed since the early 1900s (Table Hy01, Figure Hy01 ). There are six active and three inactive gage stations located in the Missouri portion of the watershed and nine active and twenty-one inactive gage stations in the Arkansas portion of the watershed. Specific information from each station, for the period of record, can be found in annual Water Resources Data Reports published by the USGS in Rolla, MO and Little Rock, AR.
PERMANENT AND INTERMITTENT STREAMS
There are many streams in the watershed which are considered intermittent for all or part of their length. The total mileage for intermittent streams with permanent pools is 210.5 miles for the Missouri portion of the watershed. The length of streams with permanent flow is 298.5 miles (Funk 1968). Intermittent streams are represented as broken blue lines on USGS 7.5 minute topographic maps, while permanent streams are represented with solid blue lines. Figure Hy02 shows coverage of USGS 7.5 minute topographic maps for the watershed, and map names are listed in Table Hy02.
Losing stream reaches, streams that lose portions or all of their surface flow to underground flow, are listed in Table Hy03 for the Missouri portion of the watershed. Losing streams are direct links between surface water and groundwater and have the potential to transfer undesirable contaminants to groundwater.
Base flows in most streams are well sustained during dry weather, due in part to the high storage capacity of the regional geology, coupled with favorable precipitation and runoff conditions. Springs help sustain flow in many watershed streams. There were 104 watershed springs identified from USGS 7.5 minute topographic maps. Springs in the watershed are listed in Table GE01and displayed in Figure GE02. The largest spring in the Missouri portion of the watershed, for which flow has been determined, is Roaring River Spring, with an average daily flow of 20,400,000 gallons and a maximum recorded daily flow of 114,000,000 gallons (Vineyard 1982).
Instream flow refers to the quantity of water, and its variation over time, as it exists in a watercourse, also referred to as flow regime. Some instream flow uses in the watershed include, protection of aquatic organisms, hydroelectric power production, recreation, channel maintenance, and transport of effluent discharges.
The 7-day Q2 and Q10 values represent the relative permanence of a stream. The 7-day low flow discharges, with recurrence intervals of two years (Q2) and ten years (Q10), for locations throughout the watershed are found in Table Hy04. The minimum recorded flow from Beaver Dam is 47 cubic feet per second (cfs) and from Table Rock Dam is 100 to 110 cfs (MDNR 1996b). Table Hy05 gives historic high and low flow information for several gage stations throughout the watershed. Base flows are maintained by springs and, even during the driest periods, watershed streams have some of the best maintained base flows in Missouri. The high relief of the watershed results in rapid runoff during periods of heavy rain, and stream levels can increase rapidly.
DAMS AND HYDROPOWER INFLUENCES
There are three large hydroelectric dams on the mainstem White River; Beaver Dam, Table Rock Dam, and Bull Shoals Dam. All three are owned by the USCOE and electricity is distributed by the Southwestern Power Administration (SWPA). The dams were constructed and are operated for flood control and to provide electric power, with an added authorization of Bull Shoals Lake to provide water for municipal and industrial uses. Much discussion has taken place concerning a reauthorization of the White River Reservoir System to include recreation and natural resources in the stated purposes of the lakes. Powersite Dam is a considerably smaller mainstem hydroelectric dam owned and operated by the Empire District Electric Company (EDEC).
The uppermost mainstem hydroelectric dam is Beaver Dam near Eureka Springs, AR at river mile (RM) 609.0. Beaver Lake was formed in 1963 with the closing of Beaver Dam. Beaver Lake has a conservation pool elevation of 1,120 feet above mean sea level (msl) and a flood pool elevation of 1,135 msl. Beaver Lake contains 28,220 surface acres of water at conservation pool and 31,700 acres of surface water at flood pool. Beaver Lake impounds 37 miles of the White River.
The first mainstem hydroelectric dam in the Missouri portion of the watershed, and second in line below Beaver Dam, is Table Rock Dam near Branson, MO, located at RM 528.8. Table Rock Dam was closed in June of 1959, and Table Rock Lake impounds approximately 80 miles of the mainstem White River. Table Rock Lake’s conservation pool elevation is 915 feet msl, and the flood pool elevation is 931 feet msl. Table Rock Lake, at conservation pool, has 39,652 surface acres in Missouri and 3,448 surface acres in Arkansas. Table Rock Lake at full flood pool impounds water to within about 3 miles of Beaver Dam.
The next mainstem dam is Powersite Dam located at RM 506.1. Powersite Dam is a considerably smaller mainstem hydroelectric dam owned and operated by EDEC. Powersite Dam was closed in 1913 creating 2,080-acre, Lake Taneycomo. Lake Taneycomo impounds 22 miles of the White River, and the top of the overflow dam has an elevation of 701.2 feet msl. Water releases from Table Rock Dam vary hourly and daily and keep Lake Taneycomo in a somewhat riverine state.
Bull Shoals Dam is the next in the series of mainstem hydroelectric dams, located near Mountain Home, AR. Bull Shoals Dam was closed in 1952 impounding 86 miles of the White River and creating Bull Shoals Lake. Bull Shoals Dam is located at RM 418.6. Bull Shoals has a conservation pool elevation of 654 feet msl and flood control elevation of 695 feet msl. Bull Shoals Lake at conservation pool covers 16,335 surface acres in Missouri and 29,105 surface acres in Arkansas.
The most obvious impact these dams have had on the White River is the inundation of the total Missouri length of the mainstem White River and the loss of habitat and aquatic fauna associated with this type of riverine system. Reservoir construction has also had a negative impact on lower stretches of tributary streams by altering flow regimes and negatively impacting riparian vegetation and aquatic life. Specific examples of species losses attributed to reservoir construction are dealt with in the biotic section.
Cold water releases from the three large mainstem dams have drastically altered the warmwater fisheries that once existed in the mainstem White River. The water released from the hypolimnion of the reservoirs is colder than that which once sustained the native fishery. These temperature changes have had the most noticeable impact in stream reaches closest to the dams, but less obvious impacts have been observed through the entire White River system, to its confluence with the Mississippi River (Shirley 1992).
Directly below each of the three major dams, coldwater fish species have been introduced and now replace the native warmwater species. Congress authorized the building of the Norfork National Fish Hatchery in 1956 as partial mitigation for the lost warmwater fishery (Patterson 1993). As a result, trout have been stocked in the tailwaters of the three dams and a put-and-take trout fishery has existed since that time. A study conducted on Lake Taneycomo compared fish populations between the pre-Table Rock warmwater conditions and the post-Table Rock coldwater conditions (Table Hy06). Lake Taneycomo as a warmwater fishery had standing stocks that included largemouth bass (8.7%), crappie (4.6%), other sunfishes (13.2%), and catfishes (9.7%) (percents indicate species percent of total standing stock). At the time of the survey no trout were present. Within nine years of the impounding of Table Rock Lake and the ensuing coldwater release, trout made up 95% of the harvest in Lake Taneycomo (Shirley 1992).
Low dissolved oxygen levels in the tailwaters of the three major dams (Beaver, Table Rock, and Bull Shoals dams) has also had negative impacts on the introduced coldwater fisheries. Increased nutrification from human and agricultural sources has spurred lower dissolved oxygen levels in the hypolimnion of the reservoirs (USGS 1995). The dams are all bottom release structures, and low oxygen levels in the tailwaters have caused problems for the introduced fishes which are very sensitive to low dissolved oxygen levels. The greatest potential for low dissolved oxygen problems occurs from July through December as the lakes stratify into distinct layers. A cooperative effort between SWPA, USCOE, EDEC, AG&FC and MDC has tried several methods to improve dissolved oxygen levels below the three large dams.
Instream flow, affected by the four mainstem dams, is a major issue in the watershed. Instream flow affects the availability of aquatic habitat, dissolved oxygen levels, and angling opportunities. Operation of Beaver and Bull Shoals dams, in Arkansas, and Table Rock Dam in Missouri substantially alter stream flows in the White River system. Hydroelectric peaking operation at these dams results in rapid changes in flow, extremely low flows, dewatered substrate, reduced fish and invertebrate habitat, and low tailwater dissolved oxygen levels, all of which can prove detrimental to fish and invertebrate populations. A study conducted by MDC (Lobb, Kruse, and Roell 1997) found that substantial increases in aquatic habitat in the tailwater section of Lake Taneycomo, directly below Table Rock Lake, would result from moderate increases in the normal flow release of Table Rock Dam. Recommendations for improving stream flow management at Table Rock Dam have been forwarded to the USCOE. Additional research is needed to refine these recommendations and fully document the benefits to the aquatic community and the Lake Taneycomo recreational fishery. Similar efforts to study and improve stream flow management at Bull Shoals are further along for the Arkansas portion of the White River. Further interstate efforts to establish minimum flows below these dams are ongoing, and cooperation between MDC, AG&FC, USCOE and SWPA remains critical to finding better ways to manage flows and protect the downstream fisheries (Lobb D., MDC memo, 1998).