Page 294 - Ultimate Visual Dictionary (DK)

GEOLOGY, GEOGRAPHY, AND METEOROLOGY

Lakes and groundwater

NATURAL LAKE OCCUR WHERE a large quantity of water collects in a hollow EXAMPLES OF SPRINGS
in impermeable rock, or is prevented from draining away by a barrier, such
Permeable Water table
as moraine (glacial deposits) or solidified lava. Lakes are often relatively limestone
short-lived landscape features, as they tend to become silted up by sediment
from the streams and rivers that feed them. Some of the more long-lasting Spring
line
lakes are found in deep rift valleys formed
by vertical movements of the Earth’s crust
Stream
(see pp. 58-59)—for example, Lake Baikal
in Russia, the world’s largest freshwater
lake, and the Dead Sea in the Middle East,
one of the world’s saltiest lakes. Where Spring Impermeable
water is able to drain away, it sinks into shale
the ground until it reaches a layer of LIMESTONE SPRING
impermeable rock, then accumulates in Permeable Water
the permeable rock above it; this water- gravel table
saturated permeable rock is called an
Spring
aquifer. The saturated zone varies in line
depth according to seasonal and climatic
LAKE BAIKAL, RUSSIA
changes. In wet conditions, the water
stored underground builds up, while in dry periods it becomes depleted.
Where the upper edge of the saturated zone—the water table—meets Stream
the ground surface, water emerges as springs. In an artesian basin, where the Spring Impermeable
aquifer is below an aquiclude (layer of impermeable rock), the water table clay
throughout the basin is determined by its height at the rim. In the center COASTAL (VALLEY) SPRING
of such a basin, the water table is above ground level. The water in the Fault Water
basin is thus trapped below the water table and can rise under its own Permeable table
sandstone
pressure along faultlines or well shafts.
Spring line

STRUCTURE OF AN ARTESIAN BASIN Recharge area Spring

Water table
Stream
Height of water table Impermeable
in recharge area shale
FAULT SPRING
Water
Spring table
line
Jointed,
Artesian solidified
spring
lava
Spring
Aquiclude
Aquifer (impermeable
Aquiclude (saturated rock) Jointed,
(impermeable Artesian rock) solidified Stream Impermeable
rock) well lava mudstone
Artesian Fault
spring LAVA SPRING

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