The Science of Instream Flows: A Review of the Texas Instream Flow Program (2005)

The Texas instream flow program exists within scientific and program contexts. The scientific context includes the state’s hydrologic, physical, and climatic settings; the program context includes water management statutes and Texas water programs. This chapter describes these contexts of the Texas instream flow program.

Texas climate and topography exhibit great variations across its vast 266,805 square miles. Topography across the state includes flat, level plains of the Texas Panhandle, basins and mountains in the Trans-Pecos region, and rolling hills in east Texas. Western areas of the state are dry and the coastal areas in the east are humid. Texas’ wide span of hydrologic and physical riverine conditions impacts how instream flow science is conducted across the state.

Precipitation ranges from an average of 8 inches per year in far West Texas to as much as 60 inches per year in coastal east Texas (TPWD, TCEQ, and TWDB, 2003). Texas rivers reflect this precipitation variability: rivers in west Texas generally exhibit greater seasonality in flows and a higher frequency of flash floods, and rivers in east Texas generally carry higher flows with less seasonal variation. Many of the state’s streams and rivers flow from the north and west toward the south and east (see Figure 1-1). Texas, more than other states in the United States, has a hydrological regime with a high flash-flood potential (Beard, 1975). This potential varies across the state from west to east, like the river drainage basins themselves

(see Figure 1-1), and it is an important consideration in the hydrology, hydraulics, and aquatic ecosystems in Texas rivers.

Differences in hydrologic regime across the state have important implications for instream flow science. For example, in the flood-dominated river basins of central and west Texas, geomorphic dynamics do not conform to the classic “equilibrium” concepts of geomorphology, even when land use change, channelization, or dam construction have not occurred in the watershed (Baker, 1977). Therefore, geomorphological dynamics of rivers in Texas follow a west-east spatial gradient across the state from disequilibrium behavior in the west to equilibrium-like behavior in the east. This strong hydro-geographic gradient is reflected in the physical structure of aquatic and riparian habitat and other ecological processes and patterns.

The physical setting of Texas rivers can be described in many ways for the wide variety of conditions across the large state. For descriptive, instream flow purposes, the state of Texas and its river systems are coarsely categorized into five generalized districts: East, North-Central, South-Central, Lower Rio Grande basin, and West. These districts are described briefly below in terms of geology, climate, hydrologic regime, and biota.

East Texas rivers (Lower Red, Lower Trinity, Lower Brazos, Navasota, Sabine, Neches) drain the portion of Texas with average rainfall between 30 and 50 inches a year. The region is dominated by flat landscapes and either clay-rich or sandy soils (the latter associated with the Sabine and Neches watersheds). Rivers of this region historically experienced periodic flood pulses that connected river channels to floodplains. Watersheds of the region are dominated by agriculture. The 1950s were a period of dam construction across this region, and today most major rivers have been impounded for flood control purposes. Water-based recreation is popular in this region, especially fishing in some of the state’s largest and most productive reservoirs. The region contains several imperiled aquatic species, including paddlefish (Polyodon spathula) and sharpnose shiner (Notropis oxyrhynchus). Fish communities in east Texas (in basins like the Brazos River) are dominated by species adapted to high variations in flow, high turbidity (especially in the Trinity, Brazos, and Red drainages), and harsh environmental

conditions. Channel substrates are mostly soft, shifting sediments (sand, mud, and silt). The dominant physical structure within stream channels is woody debris.

North-central Texas rivers (Canadian, Upper Brazos, Upper Colorado, Upper Red, Upper Trinity) drain watersheds with clay-rich soils and heavy agriculture use. This region is drier than east Texas, with rainfall averaging between 15 and 28 inches a year and occasional severe droughts. These rivers have flow characteristics similar to those of East Texas rivers, but they are smaller and tend to experience more frequent drought conditions. The region is dominated by fish species that are resistant to alternating drought and flood conditions. Like east Texas, water-based recreation is also quite popular. This region includes several of the state’s large metropolitan areas (Dallas/Forth Worth, Amarillo, Lubbock, Waco).

This region is better known as “The Hill Country” of Texas. Rivers that drain this region include the Blanco, Comal, Frio, Guadalupe, Lower Colorado, Nueces, Sabinal, San Antonio, and San Marcos rivers. The landscape in this region is rocky in many areas, and the dominant land use in this region is livestock grazing. The region, which includes the Edwards Plateau, has a relatively wide range of average annual precipitation. Parts of this region are relatively dry and experience periodic drought, with average annual rainfall of around 10 inches, other parts receive up to about 40 inches a year. Rivers in the region receive significant subsurface flow and tend to flow clear and cool most of the time, but experience relatively infrequent flash floods during spates. The region harbors several threatened and endangered fishes including the fountain darter (Etheostoma fonticola), Clear Creek gambusia (Gambusia heterochir), and cave catfishes (Satan eurystomus, Trogloglanis pattersoni). Two of the state’s fastest growing metropolitan areas, Austin and San Antonio, are located along this region’s eastern border. The rapid population growth of these two urban areas has placed sharp demands on the region’s limited water resources. Hill Country rivers and streams are used for a variety of recreational purposes, including swimming, rafting, canoeing, and fishing.

The largest rivers in south Texas are the Lower Rio Grande River and its tributaries, including Devils River, Las Moras Creek, and San Felipe Creek. Annual average rainfall in this region varies from 11 to 26 inches. The region’s prevalent land uses are irrigated row cropping in the Lower Rio Grande Valley, and livestock grazing across the region. The region’s major cities are Brownsville, McAllen, and Del Rio. Population growth in this region also is exerting increasing pressure on limited water resources. Over the past several decades, instream flow in the Lower Rio Grande has been progressively reduced by upstream water diversion, withdrawal, and evaporation from reservoirs. Today, the Lower Rio Grande channel is periodically reduced to a series of isolated pools, and the river fails to reach the Gulf of Mexico for extended periods. The Rio Grande is an extreme example of how aquatic biota evolutionarily adapted to pre-Columbian stream flows have been stressed to the point of being detrimental to their survival by changes and disruptions to natural flows. Threatened and endangered aquatic species in this region include the Devils River minnow (Dionda diaboli), prosperine shiner (Cyprinella proserpina), and Rio Grande darter (Etheostoma grahami). Water-based recreation use is increasing on those rivers that have more reliable year-round flows.

West Texas is the driest region of the state. Some areas of west Texas receive annual average rainfall of roughly 16 inches, but that figure falls to less than 8 inches in far west Texas. The region’s aridity has resulted in strong pressures on its surface and groundwater resources. The principal land use is livestock grazing, especially for sheep and goats. The principal rivers in this region are the middle Rio Grande and the Pecos. The Pecos River is highly saline and has experienced golden algae blooms that kill fish and other aquatic animals (Rhodes and Hubbs, 1992). Endangered aquatic species are a common occurrence and include the Comanche Springs pupfish (Cyprinodon elegans), Leon Springs pupfish (C. bovinus), Pecos pupfish (C. pecosensis), and Pecos gambusia (Gambusia nobilis).

Climate, which controls hydrology and affects all other aspects of a river system, is a critical element in water planning in Texas (TWDB, 2002a). The U.S. National Assessment of climatic change impacts on the U.S. (U.S. National Assessment Synthesis Team, 2001) reports that the southern Great Plains is likely to experience increases in temperature, frequency of heat-stress events, and precipitation changes with a shift toward more intense rainfall events, and frequency and severity of droughts. The result is “expected to exacerbate the current competition for water among the agricultural sector, urban and industrial users, recreational users, and natural ecosystems, as well as within each user community” (U.S. National Assessment Synthesis Team, 2001).

In 1840, Texas adopted the riparian doctrine that entitles a landowner to a reasonable use of river water that abuts his or her land. Thus, for much of the last half of the nineteenth century, land ownership determined the right to use river water in Texas. A series of state laws adopted at the end of the nineteenth century declared that then unappropriated surface waters were to be the property of the public and that future rights to use water would be acquired under a prior appropriations system. While Texas continued to honor pre-existing riparian rights, this new legal structure set a very different course for Texas.

Under the prior appropriation doctrine, the most senior water right holder is entitled to have his or her water right fully satisfied in times of shortage, before the next most senior water right holder is allowed to divert or store water. Thus, the prior appropriation system often is described as “first in time, first in right.” The prior appropriation system typically limits appropriators to the diversion or storage of water for “beneficial uses,” a term which has evolved in Texas and the western states as societal expectations have changed (for example, many western states now treat non-consumptive instream uses to be beneficial). Unlike the law of riparian rights, the law of prior appropriation also allows a water rights holder to divert water from a stream and to transport the water to be used at some distance from the stream, potentially even in a different basin. Finally, in Texas and other western states, a water right can be lost through non-use over a period of years.

Texas embraced this dual system of water law—riparian rights coexisting alongside appropriative rights—until it was merged with prior ap-

propriation with the Water Rights Adjudication Act. The 1967 legislation merged the riparian system with the prior appropriation system that controls water allocation in most of the West by mandating a comprehensive adjudication of all water rights on individual river and stream segments. All pre-existing water rights that could be proved up were granted certificates of adjudication while future applicants were required to secure a permit, known as a water right, from the state agency, now the Texas Commission on Environmental Quality (TCEQ), charged with administration of the permit system. Since the inception of the current permit system, the TCEQ has also had the authority to grant, deny, and condition water rights to best serve the public interest.

Several statutes adopted in 1977 expanded the TCEQ’s ability to protect environmental values, including instream values, as part of its mandate to manage and allocate water resources in Texas. A state statute gave the commission the authority to maintain a proper ecological environment in the state’s bays and estuaries for permits issued within 200 miles of the coast and the ability, when practicable, to include conditions that are necessary to maintain beneficial inflows to bays and estuaries. Other provisions allowed the commission, when it weighs applications for new and amended permits, to consider a diversion’s effects on existing instream uses, water quality, and fish and wildlife habitat.

Prompted by a serious drought as well as dramatic projections of population growth, in 1997 the Texas legislature adopted a sweeping reorganization of its water resource management regime. The legislation, known as Senate Bill 1, mandated that the Texas Water Development Board (TWDB), TCEQ, and the Texas Parks and Wildlife Department (TPWD) work together to produce a state water plan that was, in turn, to be based upon water plans developed by sixteen regional planning organizations along with the TWDB’s analysis and policy recommendations. The first state water plan, based largely upon the regional plans, was completed in 2002 and the state is now engaged in a second round of planning to refine that plan. Senate Bill 1 also enacted new provisions regarding a wide range of other difficult issues, including groundwater management, inter-basin transfers, reuse, water marketing, and cancellation of water rights for non-use. Finally, Senate Bill 1 also was described by then-Lieutenant Governor Bullock as having recognized that “water must be available to satisfy environmental needs for Texas’s fish and wildlife habitat, instream flows, bays,

and estuaries. This legislation adds numerous new provisions that require environmental water needs to be considered whenever a water supply is developed, transferred, reused, or marketed.”

Four years later, in 2001, the Texas state legislature adopted another piece of legislation, Senate Bill 2, dealing extensively with water law. This legislation established a Water Advisory Council, and made a number of technical changes to the state’s water code. Of particular importance for this report, the legislature also directed the three resource agencies—TCEQ, TWDB, and TPWD to:

• Jointly establish and maintain an instream flow data collection and evaluation system;

• Conduct studies to determine appropriate methodologies for determining flow conditions in the state’s rivers ands streams necessary to support a sound ecological environment;

• Complete priority studies by December 31, 2010; and

• Direct the Commission to consider the results of the studies in its review of management plans, water rights, and interbasin transfers

Finally, it is important to note that much of the State of Texas’s groundwater resources are managed under an allocation regime that is largely separate and distinct from the prior appropriations system used for surface waters. Texas law presumes that all underground water sources are “percolating waters,” which are subject to the English common law doctrine known as the Rule of Capture. Within this legal framework, the land owner is generally permitted to withdraw water (to reduce water to his or her possession) from these underground sources even if the withdrawals deleteriously affect the quantity of water found beneath an adjacent surface property or diminishes the flow of surface streams. However, that rule is subject to limitation if the withdrawal causes malicious injury to another landowner, or causes willful waste.

Moreover, the Texas state legislature has established some general as well as some site-specific statutory constraints on the operation of the Rule of Capture. At the site-specific level, Texas has established a coastal subsidence district to limit pumping from the Gulf Coast aquifer in the Galveston region, and an aquifer authority which is charged with managing groundwater withdrawals from the Edwards aquifer to protect endangered species and maintain flow levels in the Guadalupe River. More generally, the Texas state legislature in Senate Bill 1 reaffirmed that local groundwater conservation districts are principally responsible for managing groundwater resources. It also provided the districts with more statutory authority than

had previously been available. In addition, the Texas state legislature required these entities to develop groundwater management plans and the TWDB to certify the plans once they are complete. There are now almost eighty groundwater districts in Texas, most of which were created along county rather than aquifer boundaries.

There are three Texas programs that deal with water availability and water quality that are directly relevant to conducting instream flow studies and implementing the results of those studies. Water availability is important because the amount of water in streams that remains available for allocation will have important effects on the ability of the Texas agencies to implement any flows recommended as a result of an instream flow study. The water quality programs are specifically mentioned in the Technical Overview Document (TOD) as producing information that should be considered in designing and conducting an instream flow study. These programs are briefly described below.

Permits are required to withdraw water from Texas streams and rivers. These permits are administered by the Texas Water Rights Permitting Program within the TCEQ. In response to Senate Bill 1, TCEQ made substantial improvements in the rigor of its evaluation of permits for surface water withdrawals by developing a water availability model (WAM) for each of the twenty-three river and coastal basins of Texas. That model is used both in the permitting process and in regional water planning. The input data set for the WAM identifies a set of control points where a control point is a diversion or storage location on a river, and includes physical data about each control point, such as the amount of permitted withdrawal, upstream drainage area, and the priority date. Also included in the input data set is a sequence of “naturalized flows” derived from United States Geological Survey (USGS) stations (the naturalized flow is a modeled flow which would have occurred if no diversions or upstream reservoirs existed). Typically, this flow sequence is defined for monthly flows and encompasses 40-50 years of historical data.

It appears that many permits have not yet been exercised, or have not yet been exercised in full. Water remains available for allocation in many

streams and rivers across Texas. TCEQ defines the limits of water withdrawals from rivers using a specific percentage of naturalized flow as the minimum flow. Generally, water is less available in the upper parts of the basins than in the lower basins.¹

The State of Texas implements the National Pollutant Discharge Elimination System (NPDES) program through the Texas Pollutant Discharge Elimination System (TPDES) and it applies to all point sources, including municipal effluent. In many cases, the agency must conduct a receiving water assessment before it can issue a TPDES permit, and Texas uses the QUAL-TX² model to provide analytical support for the TPDES program. This water assessment process using QUAL-TX creates conservative estimates of pollutant loads because QUAL-TX accounts directly for point-sources and indirectly for non-point sources. The biological studies needed to support the Texas instream flow program effectively require collection of much the same kind of data as is needed to complete a receiving water assessment of a wastewater discharge. However, the receiving water assessment is focused on the point of discharge and resulting effects downstream, while an instream flow study may need to characterize the entirety of a long river reach.

The Texas Water Quality Inventory is prepared by TCEQ and submitted to the Environmental Protection Agency (EPA) biannually in even-numbered years in accordance with section 305(b) of the Clean Water Act. Water bodies that do not support their water quality standards and for which existing controls are not adequate are placed on the 303(d) list of impaired water bodies, and then come under the domain of the total maximum daily load (TMDL) program (discussed in detail in Chapter 5). For aquatic life use, the criteria include dissolved oxygen, physical habitat, toxic substances in water and sediment, and biological assessments, though ade-

quate data to reach conclusions on these assessments are available for only a portion of all the water bodies in the state, especially for toxic substances and biological assessments. Aquatic life in Texas streams and rivers is to some degree impacted by toxic chemicals but the main water quality limitation on aquatic life is depressed dissolved oxygen.