Ouachita
Mountains (Ecoregion 36) The
Ouachitas are made up of ridges, hills, and valleys formed
by the erosion of folded and faulted Paleozoic sandstone, shale,
and chert, known locally as novaculite. They are a continuation
of the Appalachians, formed during the late Paleozoic Era when
an ocean closed and continents collided, causing marine sediments
to be folded, faulted, and thrust northward. The Ouachitas
are structurally different from the Boston Mountains (38),
more folded and rugged than the lithologically distinct Ozark
Highlands (39), and physiographically unlike the Arkansas Valley
(37), South Central Plains (35), and Mississippi Alluvial Plain
(73).
Potential
natural vegetation is oak-hickory-pine forest; it contrasts with the
oak-hickory forest that dominates Ecoregion 39 and the northern part
of Ecoregion 38. Today, loblolly pine and shortleaf pine grow in a
distinctive mix of thermic Ultisols and Inceptisols. Logging and recreation
are major land uses, and pastureland and hayland are found in broader
valleys.
Regional
water quality is influenced by lithology, soil composition, and land
use activities. In most reaches, water quality is exceptional; typically,
total phosphorus, turbidity, total suspended solids, and biological
oxygen demand values are lower whereas dissolved oxygen levels are
higher than in Ecoregions 35, 37, and 73. Water hardness varies by
level IV ecoregion; Ecoregions 36d and 36e tend to have the lowest
hardness values while progressively higher values occur in Ecoregions
36a, 36b, and 36c. Stream substrates are made up of gravel, cobbles,
boulders, or bedrock; they contrast with the fine-grained substrates
of lower gradient streams in Ecoregions 35 and 73. The fish community
is dominated by sensitive species; minnows and sunfish along with darters
and bass are common.
The
Ouachitas are made up of ridges, hills, and valleys formed
by the erosion of folded and faulted Paleozoic sandstone, shale,
and chert, known locally as novaculite. They are a continuation
of the Appalachians, formed during the late Paleozoic Era when
an ocean closed and continents collided, causing marine sediments
to be folded, faulted, and thrust northward. The Ouachitas
are structurally different from the Boston Mountains (38),
more folded and rugged than the lithologically distinct Ozark
Highlands (39), and physiographically unlike the Arkansas Valley
(37), South Central Plains (35), and Mississippi Alluvial Plain
(73). 
Potential
natural vegetation is oak-hickory-pine forest; it contrasts with the
oak-hickory forest that dominates Ecoregion 39 and the northern part
of Ecoregion 38. Today, loblolly pine and shortleaf pine grow in a
distinctive mix of thermic Ultisols and Inceptisols. Logging and recreation
are major land uses, and pastureland and hayland are found in broader
valleys. 
Regional
water quality is influenced by lithology, soil composition, and land
use activities. In most reaches, water quality is exceptional; typically,
total phosphorus, turbidity, total suspended solids, and biological
oxygen demand values are lower whereas dissolved oxygen levels are
higher than in Ecoregions 35, 37, and 73. Water hardness varies by
level IV ecoregion; Ecoregions 36d and 36e tend to have the lowest
hardness values while progressively higher values occur in Ecoregions
36a, 36b, and 36c. Stream substrates are made up of gravel, cobbles,
boulders, or bedrock; they contrast with the fine-grained substrates
of lower gradient streams in Ecoregions 35 and 73. The fish community
is dominated by sensitive species; minnows and sunfish along with darters
and bass are common.