QUALITY, SURFACE WATERS, NECHES RIVER BASIN, TEXAS
The Neches Eiver drains an area of about 10,000 square miles in eastern Texas (fig. 1). The basin is about 200 miles long, averages about 50 miles wide, and includes all or part of 21 counties. From its source in southeast Van Zandt County (fig. 2), the Neches Eiver flows generally southeastward and empties into Sabine Lake, an arm of the Gulf of Mexico.

Low divides separate the Neches Eiver basin from the Sabine Eiver drainage basin on the north and east and from the Trinity Eiver drainage basin on the west and southwest. The basin slopes from an altitude of about 600 feet to sea level. The northwestern third of the basin has rolling hills and grassy plains. The area from central Cherokee County southward to south- ern Hardin County consists of heavily forested low hills and wide flat flood plains along the Neches Eiver and its major tributaries. Southern Hardin County and Jefferson County have prairies and poorly drained flatlands.

The Neches Eiver basin is drained by two major streams and many tributaries. The Angelina Eiver heads in southwest Eusk County and, at Dam B Eeservoir, joins the Neches River. Upstream from their confluence, the Neches Eiver drains 3,808 square miles and the Angelina Eiver drains 3,556 square miles. Village Creek and Attoyac and Pine Island Bayous, with drainage areas of 1,113, 670, and 657 square miles respectively, are the only other tributaries that drain more than 500 square miles. The climate in the Neches River basin ranges from moist subhumid to humid. The average annual precipitation, about 49 inches, exceeds the average for the State of Texas by 60 percent. Within the basin, the average annual precipitation ranges from about 46 inches in the northwest to more than 54 inches in the southeast. At Rockland, in Tyler County, annual rainfall for the period 1931-60 averaged 49.85 inches. Mean annual precipitation, average (normal) monthly pre- cipitation of four Weather Bureau stations, and annual precipitation for 1910-63 at one station are shown on figure 2.

Runoff is defined as that part of the precipitation appearing in surface streams, and is the same as streamflow unaffected by artificial storage or diversion (Langbein and Iseri, 1960, p. 17). Streamflow in the Neches River basin has been affected only slightly by diversions or storage. Temperature, seasonal distribution of rainfall, storm inten- sity, infiltration rates, and types and density of vegetation also affect the amount of runoff from a drainage basin. Runoff data plotted on figure 2 show that average runoff from sub- basins during the period 1940-63 has ranged from 8.9 to 13.1 inches annually. Runoff from the entire basin measured at the lowermost gaging station, Neches River at Evadale, averaged 11.0 inches an- nually for the period 1921-63. Annual runoff, expressed as mean dis- charge in cubic feet per second and inches per year, in shown for the Evadale station on figure 2.

Precipitation and runoff in the Neches River basin are subject to much greater variations than indicated by the annual and monthly averages. The yearly mean discharge of the Neches River at Eva- dale has ranged from 994 to 12,700 cfs (cubic feet per second), as shown in figure 2, but instantaneous flows have varied much more widely. Normal monthly rainfall at Rockland ranges from 2.88 inches for August to 5.39 inches for January (fig. 2), but in 1963 the monthly rainfall ranged from 0.00 inches in October to 8.10 inches in September. Thus, in spite of relatively high averages, precipitation so unevenly distributed in time does not sustain streamflow, and flood runoff must therefore be stored to make surface water continuously available in dependable quantities.