The following, though dated, may suggest a pattern, suggest historical conditions, and allow comparisons of past information with current data. Orders of magnitude are likely to be the same.

Ohio

The Ohio Water Resource Region, with an area of about 420,000 km² (163,000 sq miles), includes the Ohio River and tributary systems (such as the Allegheny, Monongahela, Cumberland, Kanawha, Kentucky, Green, and Wabash rivers). The region includes scattered small natural lakes and moderate-sized reservoirs.

Surface waters of the Ohio Water Resource Region range from moderately hard (60 to 120 mg/liter hardness as CaCO₃) to hard (>120 mg/liter). The softer waters are generally found in the southern and eastern portions of the region. Surface-water hardness in parts of Illinois. Indiana, and Ohio falls in the 180 to 240 mg/liter range. Total dissolved solids levels also reflect this distribution, with lowest values (≪120 to 350 mg/liter) found in the southern and eastern portions of the region, and the highest levels (>350 mg/liter) in parts of Illinois, Indiana, Ohio, and western Pennsylvania. Total suspended solids (TSS) levels generally vary from less than 270 mg/liter to almost 2000 mg/liter. The southern Illinois area has the highest TSS levels. exceeding 1900 mg/liter.

Water quality in the Ohio basin has suffered from both point-source and nonpoint-source pollutants. Agriculture (contributing pesticides, nutrients, and sediments), construction, urban runoff, stream channelization. and industrial and municipal discharges have degraded water quality in terms of bacterial contamination. nutrients, heavy metals, and other toxicants. The region has been categorized as having major thermal pollution problems, although not as severe as those of the New England and Mid-Atlantic regions. The Ohio region has been particularly affected by coal mining drainage, with abandoned mines and the diffuse nature of the discharge exacerbating the situation. Coal mining in eastern Kentucky and the Cumberland region in Tennessee have resulted in acid mine drainage. Streams in mining areas of eastern Kentucky have suffered from elevated levels of dissolved solids, hardness. sulfate, acidity, and chloride. Even where neutralized mine wastes have been discharged, increased dissolved solids and sulfate levels have been reported. Although identifiable point- source discharges such as industrial and municipal wastes have begun to be controlled with waste treatment facilities (with observed recovery of water quality), and mine drainage has been identified as less amenable to control with "best available treatment" technologies. Portions of the Ohio have showed a small-watershed hydrologic response (see section on the Mid- Atlantic region) ranging from stable (<4%) to "flashy" (>24%). The watersheds with the most rapid flow response to precipitation were found in northern Kentucky.

Approximately 4.09 x 10¹¹ m³ (108,000 x 10⁹ gal) of potable groundwater is available from storage in the aquifers of the Ohio region. Consolidated rock and minor unconsolidated aquifers contain 3.2 x 10¹¹ m³ (85,000 x 10⁹ gal) of the total amount; outwash and alluvial aquifers may store as much as 8.7 x 10¹⁰ m³ (23,000 x 10⁹gal) of potable groundwater (Bloyd 1974).

The groundwater reservoirs in the Ohio region are of four general types: alluvium. outwash. glaciofluvial deposits, and bedrock. Alluvium of recent age, consisting of silt, sand, and gravel, occurs in the valleys of the larger streams. Sand and gravel glacial outwash deposits are found in the glaciated valleys of the main tributaries. Glaciofluvial deposits are comprised of both Pleistocene outwash and recent alluvium; these occur within the Ohio River valley as well as along its major tributaries. Well yields from the unconsolidated aquifers can exceed 31 liters/sec (500 gpm). Thick bedrock units varying in age from Precambrian to Tertiary underlie the region. The greatest thicknesses are coincident with the Appalachian structural basin in the east and the Illinois structural basin in the west. Maximum well yields of consolidated aquifers range from 6 to 31 liters/sec (100 to 500 gpm).

Ground water quality within the region varies primarily with depth. Shallow groundwater generally contains less than 1000 mg/liter TDS. At depths greater than 150 m (500 ft), however, TDS concentrations may exceed 35,000 mg/liter.

Recharge occurs primarily by infiltration of precipitation. Some seepage from streams takes place. The average annual regional groundwater recharge is about 1.3 x 10⁸ m³/day (35,000 Mgd), which may be as much as 15% of the total annual precipitation. Discharge takes place through seepage to streams, pumpage, and underflow to adjacent areas.

The base-year (1960) use of groundwater for municipalities and rural demands was approximately 3.79 x 10⁶ m³/day (1000 Mgd). or about 3% of annual recharge. Industrial use of groundwater was also equivalent to 3% of annual recharge.

In the Ohio Water Resource Region, average annual runoff is equivalent to 474 x 10⁶ m³/day (125 Bgd). Of the total off-channel water withdrawal of about 140 x 10⁶ m³/day (36 Bgd) in 1975, surface freshwater withdrawals accounted for about 130 x 106 m³/day (34 Bgd); groundwater contributed about 7.2 x 10⁶ m³/day (1.9 Bgd). About 3% of freshwater withdrawn was actually consumed. Primary groundwater users were the industrial and public supplies sectors. The greatest user of water and consumer of freshwater in the region is the self-supplied industrial sector, including electricity-generating utilities. Generation of hydroelectric power uses approximately 870 x 10⁶ m³/day (230 Bgd). with the Ohio region ranking fourth in this respect.

Projections to the year 2000 indicate that development of energy resources (coal, oil, gas, oil shale, and tar sands) in the Ohio River basin could stress available water supplies. Monthly flow varies considerably in the Ohio River basin, and some upper mainstem and tributary areas already experience low-flow problems. Increased energy development in the Allegheny, Monongahela, Muskingum, Scioto, Miami, Hocking, and Kentucky river basins could exacerbate low-flow problems. Future energy demand has been seen as reducing the 7-day 10-year low-flow in the Ohio River mainstem by 15% in 2000. Reservoir development (in addition to the importation of power) has been identified as a possible future need. There are possible water availability problems in the Green, Monongahela, and Conemaugh river basins by 1985, with significant water availability problems on the middle and lower Ohio River mainstem projected for 2020.

The Ohio Water Resource Region contains some of the same habitats found in the Tennessee Region. Mountain streams such as those described in the section on the Tennessee region are common in some parts of Pennsylvania, West Virginia, Tennessee. Kentucky. and Ohio. Because these areas are also heavily mined for coal. many of the same types of mine drainage problems exist here. Although some of the more remote streams have escaped degradation, much of the Appalachian portion of this region has been greatly impacted. Consequently, the natural biota has been largely displaced by more tolerant forms. Thus, trout that were once abundant locally have been virtually eliminated.

The upper one-third to one-half of this region was glaciated. and most of the natural lakes occur in this area. Because of the great numbers of people living nearby, these lakes receive considerable fishing pressure and are constantly enriched with anthropogenic nutrients. Some of the more shallow lakes have become overgrown with macrophytes (as a result of eutrophication) to the extent that frequent anoxic conditions develop that cause many sensitive organisms to disappear. Some progress has been made in alleviating this problem by increased sewage treatment capabilities, weed harvesting, and in situ nutrient inactivation.

Mid-Atlantic

The Mid-Atlantic basin has an area of about 264,000 km² (102,000 sq miles) and includes major rivers such as the Potomac. James, and Rappahannook that flow into the Atlantic Ocean and coastal bays. Major reservoirs are generally not Important surface-water features, although some tributaries to the Delaware and Hudson systems are impounded. Coastal areas include the Chesapeake estuary (drowned-valley) system (Chesapeake Bay and the mouths of the Potomac. Rappahannock. York, and James rivers), Delaware Bay, the mouth of the Hudson River (including Upper and Lower New York Bay), and the extensive bays between the mainland and barrier beaches from Virginia to Long Island.

Surface waters in the Mid-Atlantic region are of medium hardness (60 to 120 mg/liter hardness as CaCO₃) in the James River basin, the lower Hudson and Delaware river basins, and portions of the upper Susquehanna and Potomac river basins. They are typically soft (<60 mg/liter hardness) elsewhere. Levels of TDS in surface waters may reach a few hundred milligrams per liter in the Mohawk-Hudson river system, in areas of eastern Pennsylvania and northern New Jersey, and in a belt including the highlands area of central Maryland and the Virginia-West Virginia border. Isolated areas in eastern Pennsylvania may have surface waters exceeding 350 mg/liters TDS. In other parts of the Mid-Atlantic region, TDS levels are generally below 120 mg/liter. Stream TSS levels, although generally less than 270 mg/liter throughout the region. may be higher (270 to 1900 mg/liter) in isolated Piedmont areas of eastern Pennsylvania, Maryland, and Virginia. The hydrologic response characteristics (the percent of annual precipitation that appears as quick flow in small streams), an index of flood potential area, generally showed a stable flow response to precipitation (a hydrologic response of <or = 8%). although some Piedmont areas had a "flashier" hydrologic response (8-20%).

Regional water quality problems include severe thermal pollution, pesticides in the lower reaches of major river basins, and heavy metals, particularly in the more northern states (New York and Pennsylvania). Other toxic pollutants of concern in the region include Kepone (James River), phenols, and PCBs.

Saltwater encroachment in coastal areas as a result of groundwater pumping may increase dissolved solids in groundwater to 3000 mg/liter.

In the Mid-Atlantic Region, the estimated amount of groundwater in storage ranges from 5.3 x 10¹¹ to 1.3 x 10¹² m³ (4.3 x 10⁸ to 1.1 x 10⁹ acre-ft) for the region. The aquifers are of four general types:

• unconsolidated coastal plain sediments;
• crystalline metamorphic and igneous rocks;
• consolidated sedimentary rocks; and
• glacial sediments.

The unconsolidated deposts of the Atlantic Coastal Plain consist primarily of sand, clay, and gravel. The sediments range in thickness from 0 to 2,458 m (0 to 8,000 ft) and are hydraulically interconnected to varying degrees. Wells completed in surficial material no deeper than 91 m (300 ft) can produce up to 126 liters/sec (2000 gpm).

The crystalline igneous and metamorphic rocks produce water from fractured and weathered zones. Well yields are typically less than 3 liters/sec (50 gpm); although yields of 0.9 liters/sec (15 gpm) are considered average, some wells produce as much as 25 liters/sec (400 gpm). The groundwater quality is excellent.

The consolidated sedimentary rock aquifers are primarily fractured sandstone and solutioned limestone/dolomite. Although some well yields from the folded and faulted rocks of the Valley and Ridge province exceed 63 liters/sec (1000 gpm), most wells produce much less. The groundwater derived from limestone is hard; alkalinity is also a problem. However, the general quality is good to excellent. The sedimentary rocks of the Appalachian Plateaus province are approximately horizontal, and the limestone and sandstone bodies are interbedded with coal seams and shales. The average water well produces from 3.8 to 12.6 liters/sec (60-200 gpm). The groundwater quality is usually good, except in the vicinity of coal-mine areas, where high iron concentrations become a problem.

Glacial outwash deposits are a significant source of groundwater in the region. Many wells are capable of producing more than 63 liters/sec (1000 gpm). Groundwater from the glacial deposits is good except locally, where hardness and iron constitute a problem.

The aquifers are recharged directly at outcrop areas or by leakage from adjacent aquifers. Discharge is primarily to streams and wells, with some interaquifer flow.

In the Mid-Atlantic Water Resource Region, average annual runoff is equivalent to 320 x 10⁶ m³/day (84 Bgd). Of the total off-channel water withdrawal of 200 x 10⁶ m³/day (52 Bgd) in 1975, surface freshwater withdrawals accounted for about 83 x 10⁶ m³/day (22 Bdg); saline water withdrawals averaged about 102 x 10⁶ m³/day (27 Bdg). Groundwater contributed less than 11 x 10⁶ m³/day (3 Bgd). About 7% of the freshwater withdrawn was actually consumed.

The primary user of water in the region is the self-supplied industrial sector, including electricity-generating utilities. Almost twice as much saline water as freshwater is used for condenser and reactor cooling. In contrast to water withdrawals. the greatest freshwater consumption is accounted for by the public supplies sector. Generation of hydroelectric power uses approximately 830 x 10⁶ m³/day (220 Bgd). the Mid-Atlantic Region being the fifth-ranking region in this respect.

Projections to the year 2000 point to probable water-supply problems. Freshwater supply shortages, currently experienced in the Delaware and Potomac rivers, will probably be a factor in the Hudson and Susquehanna river areas as well. However, coastal siting, which is necessary for saline water use (currently the practice in the Lower Hudson, Delaware, and Chesapeake Bay areas), may be limited by site availability. The Water Resources Council has categorized the area, including southeastern New York, New Jersey, Delaware, and eastern Pennsylvania. as being among the nation's most critical in terms of energy-related water-supply problems. Available water supplies may be inadequate for power generation and related cooling needs by 1985.

The Mid-Atlantic Region contains a wide variety of physiographio features such as ridge and valley topography, Blue Ridge mountain terrain, piedmont. coastal plain, and glacial plains that affect aquatic ecological resources.

Unglaciated portions of the region contain very few water bodies and virtually no natural lakes. Hot springs occur in a few areas, but their total water volume is insignificant. From an ecological standpoint, however, they are of great interest as research areas because the biotic communities they harbor are very different from those in adjacent waters. The region contains numerous highly productive estuaries--important nursery grounds for many commercial and sport fish and shellfish.

This Region receives the greatest municipal sewage loading of any region, a condition that has caused severe biotic degradation of its major rivers and of many of its lakes. Coal mining in Pennsylvania and Virginia has also caused considerable harm by eliminating trout from some areas and by virtually eliminating all biota from others. Major factors contributing to this degradation include sedimentation, acid drainage, and heavy metals input. Heavy metals from industries, pesticides, and the highest regional output of thermal effluents have contributed to the demise of the major rivers. Although few of these rivers are actively fished and recreation is curtailed on many, some recent progress has been made in reversing their degradation.

Tennessee

The Tennessee River basin has an area of about 106,000 km² (41,000 sq miles) and contains the heavily impounded Tennessee River and its tributaries from headwaters in southwestern Virginia and western North Carolina to its confluence with the Ohio River at Paducah, Kentucky. Principal tributaries to the Tennessee River include the Holston and French Broad rivers, which join to form the Tennessee River at Knoxville, Tennessee, and downstream tributaries such as the Clinch, Hiwassee, Sequatchie, Elk, and Duck Rivers.

Surface waters in the Tennessee region vary from soft (<60 mg/liter hardness as CaCO₃ ) in western and southcentral Tennessee to medium (60 to 120 mg/liter) in east Tennessee and northern Alabama and Georgia. Some headwaters in southwestern Virginia have harder water (120 to 180 mg/liter). Total dissolved solids typically range from 120 to 350 mg/liter. Waters having less than 120 mg/liter TDS drain the western slopes of the Appalachians along the Tennessee-North Carolina border. Impoundment of the Tennessee River system reduces TSS levels in the mainstem, although tributaries have TSS loads in the 270 to 1900 mg/liter range.

Elevated levels of mercury in sediments and biota have been reported from the Tennessee River in northern Alabama and from the North Fork of the Holston River. Water quality problems related to surface mining of coal have occurred in the Clinch River basin. Water quality in the French Broad River, which had elevated biochemical oxygen demand (BOD), suspended solids, and heavy metals from industrial-municipal pollutants, has improved following construction of waste treatment facilities. Hydrologic response in the Tennessee Region varied from stable (< or = 8%) to "flashy" (up to 24%).

The Tennessee Region is underlain by various rock types, several of which are productive groundwater reservoirs. An estimate 7.31 x 10¹¹ m³ (2.58 x 10¹³ ft³) of groundwater is available from storage within the region. The aquifers are of three basic types: unconsolidated sand and gravel, carbonate bedrock, and nonoarbonate bedrock (which includes crystalline rocks as well as other sedimentary bedrock).

Several unconsolidated aquifers occur within the region. Stream alluvium comprises the best aquifer in the unglaciated Appalachians, the easternmost portion of the region. Thick regolith (weathered rock) in places forms a productive aquifer where it overlies solutioned carbonate bedrock.

Carbonate rock aquifers occur within about one-half of the total area of the region. The central portion of the region is underlain by a sequence of approximately horizontal early Paleozoio carbonates. Late Paleozoic (Mississippian) carbonates are the most areally extensive aquifers in the region, being associated with the Highland Rim and Cumberland Plateau provinces. Moderately to steeply dipping carbonate bedrock aquifers occur in the thrust-faulted Valley and Ridge province to the east. The Knox Group (Dolomite) is the most significant aquifer in this portion of the region, with well yields of up to 63.1 liters/sec (1000 gpm). A few isolated areas underlain by carbonates capable of producing large amounts of groundwater occur in the Blue Ridge (Unglaciated Appalachians) province. Well yields vary greatly from one carbonate unit to another and within any individual formation. Production is dependent upon both the size and number of solution openings encountered during drilling and the presence and thickness of overlying weathered rock material.

Areas where noncarbonate aquifers are used are restricted approximately to the eastern one-half of the region. The Cumberland Plateau province is an area typified by carboniferous sequences of sandstone, conglomerate, shale, and coal, which generally have only fracture permeability. Well yields average less than 5.2 liters/sec (50 gpm). Shales and sandstones of the Valley and Ridge province to the north and east constitute poor aquifers, whereas the fractured crystalline rocks of the Blue Ridge where fractures and fault zones occur contain groundwater.

The quality of groundwater is variable, but is generally suitable for public drinking-water supplies. Four percent of well and spring analyses show TDS concentrations in excess of 1000 mg/liter.

Approximately 20 to 25% of the total precipitation within the region (9.64 x 10⁵ liters/sec or 1.53 x 10⁷ gpm) is recharged to the groundwater regime. Recharge takes place primarily through losing streams and infiltration of rainfall over an aquifer outcrop area. Discharge is to wells, springs, and gaining streams.

In the Tennessee Water Resource Region average annual runoff is equivalent to 160 x 10⁶ m³/day (41 Bgd). Of the total off-channel water withdrawal of about 42 x 10⁶ m³/day (11 Bgd) in 1975, surface freshwater withdrawals accounted for about 38 x 10⁶ m³/day (10 Bgd). whereas groundwater withdrawals contributed less than 1 x 10⁶ m³/day (0.3 Bgd). About 3% of the freshwater withdrawn was actually consumed. The primary user of water (and consumer of freshwater) in the region is the self-supplied industrial sector, including electricity-generating utilities. Generation of hydroelectric power, a system controlled by the Tennessee Valley Authority, uses approximately 910 x 10⁶ m³/day (240 Bgd). The Tennessee region ranks below only the Great Lakes and Pacific Northwest regions in this respect. Projections to the year 2000 indicate no major energy-related, low-flow, water-supply problems, primarily because of the Tennessee Valley Authority's ability to regulate streamflow.

This region generally has steep topography, and none of it has been glaciated. Hence, few natural lakes or wetlands exist in the area; the water resources are dominated by streams, the larger of which have been extensively impounded. The waters have low productivity, and total dissolved solids and hardness concentrations range from low to moderate.

Mountain streams in the area are typically clear, cold, and shallow, and harbor a diverse invertebrate assemblage. Red algae have been reported from these habitats, and rainbow, brown, and brook trout are locally abundant. Many of the higher-altitude streams remain relatively pristine, but several have been extensively polluted from acid mine drainage. This disturbance has resulted in simplification of the food chain, with predominance by the few species that are tolerant of low pH, high dissolved solids, high turbidity, and increased levels of metallic and organic toxicants.

The larger rivers have been greatly altered from their original conditions. The construction of many dams has stabilized stream flows, which formerly varied considerably during the year. and has created a long series of artificial lakes where flowing water formerly existed. The effect on the biota has been profound. For example, plankton are more numerous, and species of benthic invertebrates and fish have changed. These reservoirs provide an active warmwater sport fishery dominated by bass, sunfish, crappie, and walleye, and tailrace waters provide habitat for trout in some areas. In this region, 377,000 ha (950,000 acres) of freshwater exist for fishing.

Major effluent problems that have affected the biota of the larger streams include municipal waste, which causes nuisance plant growth, and heavy metals from industrial faciltities.

South Atlantic-Gulf

The South Atlantic-Gulf region has an area of about 700.000 km² (270,000 sq miles) and contains 24 major river systems that drain the southern Appalachian Highlands southeastward and southward toward the Atlantic Ocean and the Gulf of Mexico. It also contains smaller coastal river systems, including those of Florida. The Roanoke is a prominent river system. Major reservoirs include the John H. Kerr Reservoir. The region has an extensive oceanic shoreline, and bays are prominent at the mouth of the many rivers and behind barrier beaches.

Surface waters in the South Atlantic-Gulf region are generally soft (<60 mg/liter hardness as CaCo ₃ ) except for moderately hard (60 to 120 mg/liter) waters in northern Alabama and coastal areas in peninsular Florida. Concentrations of TSS are generally low in coastal areas (<270 mg/liter), with higher levels inland (up to 1900 mg/liter).

Surface water quality problems in the region have resulted from sediment runoff from silviculture and mining, nutrient loading of surface waters, and municipal and industrial discharges. Contamination of aquatic habitats with pesticides is a problem: use of DDT in the region has been particularly high; dieldrin and lindane have been found in surface waters. Many surface waters in Georgia and Mississippi are naturally acidic as a result of high concentrations of organic acids. Trace element contamination of surface waters has also occurred; elevated arsenic levels have been found in the Cape Fear (North Carolina) and Catawba (North and South Carolina) river drainages, and elevated cadmium concentrations have been found in interior and coastal Mississippi and Alabama. Industrial mercury pollution has contaminated the lower Tombigbee-Alabama river basin, the lower Savannah River, and waters of coastal Georgia. Saltwater intrusion into groundwater aquifers has resulted from pumping, especially in Florida. Construction and operation of waste treatment facilities have resulted in improvement of water quality in many streams. The hydrologic response of the South Atlantic-Gulf Region varied from stable (<4%) to "flashy" (up to 24%); most areas were more moderate, with 4-12% of annual precipitation in small watersheds appearing as quick-response streamflow.

Total groundwater in storage with TDS concentration of less than 3000 mg/liter is estimated at 1.7 x 10¹³ m³ (1.4 x 10¹⁰ acre- ft.). The average total groundwater availability is conservatively estimated at 3.4 x 10⁶ liters/sec (5.4 x 10⁷ gpm). The principal aquifers consist of highly permeable clastics and limestones of the Coastal Plain.

Aquifers in deposits of Cretaceous age have an areal extent of at least 112,000 km² (70,000 sq miles). Well yields as great as 694 liters/sec (11.000 gpm) have been reported; however, maximum expected yields from the Cretaceous acquifers are more on the order of 158 liters/sec (2500 gpm). The TDS concentration is less than 1,000 mg/liter.

In the South Atlantic-Gulf region, average annual runoff is equivalent to 747 x 10⁶ m³/day (197 Bgd). Of the total off-channel water withdrawal of about 160 x 10⁶ m³/day (43 Bgd) in 1975, surface freshwater withdrawals accounted for about 91 x 10⁶ m³/day (24 Bgd). groundwater withdrawals about 21 x 10⁶ m³/day (5.5 Bgd). and saline surface water about 53 x 10⁶ m³/day (14 Bgd). About 13% of the freshwater withdrawn was actually consumed. The geographic distribution of water use in the region is crucial. Although the total runoff is the largest of all the eastern regions, many of the population centers (such as Birmingham, Alabama; Atlanta, Georgia; and Charlotte, Greenville, and Winston-Salem, North Carolina) are in inland headwater areas where low-flow problems result from periodic droughts. Important river mainstems, such as those of the Alabama, Tombigbee, and Apalachicola, pass through less populated areas. This disparity is also great in southern Florida, which has large population centers (Miami and Palm Beach) and high agricultural water use, but no major rivers. Rainfall in southern Florida is both highly seasonal and variable from year to year. Because of these geographic and flow-variation problems, it has been estimated that only about 190 x 10⁶ m³/day (50 Bgd) of the runoff is currently available.

The self-supplied industrial and electricity generation utility sector is the largest user of water in the region, although irrigation is the largest consumer of freshwater. Use of saline water for condenser and reactor cooling is almost equal to that of freshwater. Generation of hydroelectric power in the region requires about 800 x 10⁶ m³/day (210 Bgd). making the region the sixth-ranking in this respect. Water supply projections for the region indicate that, although supplies are available along much of the coast, increased inland use will require water resource development and increased use in southern Florida will necessitate even greater reliance on saltwater withdrawals.

This region encompasses much of the Eastern Coastal Plain and Piedmont areas. Most of the southern part of the region is characterized by very soft water, whereas Florida (particularly the southern half) generally contains moderately hard to hard waters. Natural productivity of these waters is roughly proportional to hardness.

Rivers in the northern portion of the region typically arise as mountain streams that become warmer, slower, and more turbid in the Piedmont. After crossing the fall line, these rivers enter the Coastal Plain and become broad, slowly flowing streams with lower silt content. Many of them terminate in swamps and marshes near the coast. The natural biota of these rivers changes markedly along their length; that is, trout are prevalent in the undisturbed headwaters whereas only species capable of tolerating very high temperatures, up to 35degrees C (95F ) occur in the marshy areas near the rivers' mouths (e.g., cyprinids, centrarchids).

This region has the greatest areal extent of freshwater swamps and marshes of anywhere in the country. Additionally, much of its coastline consists of salt marshes and mangrove swamps. The inland swamps are typically devoid of large fish but contain many amphibians, invertebrates, algae, and angiosperms. Their water is typically highly colored from fulvic and humic substances and is acidic. The freshwater marshes are highly productive and usually contain clear, alkaline water; they do not harbor large fish but contain a diversity of other aquatic biota. Grasses, sedges, reeds, and rushes are the emergent macrophytes of these areas, and the community is very different from that found in more temperate locations of the country.