Sand stones: The Significance of Sand Body Architecture and Sedimentary Structures
433
structures fo rmed in the Tana River, Norway, by the
action of seasonal river ice. They included ice gouge
marks up to 1m wide and several tens of meters long,
eroded in the gravel and sand of the channel floor
and also on finer overbank sediments. A more subtle
effect included fl attening of current-formed clast
imbrication by pressure from stranded ice masses,
and scour holes arounded stranded ice blocks. Kettle
holes are depressions in river bed fo rmed by the melt
of buried ice masses. Some coarsening-upward
gravel-sand cycles in Quaternary outwash (Costello
and Walker 1972) have been interpreted as the result
of channel diversion into these depressions, which
deepen as melting takes place (Rust 1975). Some of
these criteria are illustrated in Fig. 12.3.
12.6 Sam! stones: The Significance
of Sand Body Architecture
and Sedimentary Structures
12.6.1 Fluvial Style in Sand-bed Rivers
Chaunel style in sand-bed rivers is controlled by
several factors, of which the amount and variability
of discharge, the nature of the sediment load, bank
composition, and bank stability, are among the more
important (Sect. 8.1). Some of these variables are
strongly affected by climate. Thus, humidity is the
main control on vegetation, which has an important
effect on discharge characteristics, sediment load,
and fl uvial style. In upland vegetated areas at the
present day, runoff following major rainfalls is rarely
catastrophic because the precipitation is absorbed
by soil and plants and released slowly. Also, sediment is stabilized by roots, and sediment yields are
relatively low. Therefore, as noted in the previous
section, mass-flow processes are less common in
humid climatic settings. Vegetated banks are highly
resistant to erosion, as demonstrated in an experiment by Smith (1976). This limits the supply of sediment to the river, and also reduces the bank
widening that is necessary for braiding to develop. In
vegetated areas, therefore, because of the effects on
discharge variability, bank stability, and sediment
supply, braiding is inhibited. Baker (1978a) quoted
examples in tropical Africa and South America
where dense bank vegetation inhibits braiding, even
where abundant coarse sediment is present in the
banks. Stanistreet et al. (1993) described the channels of the Okavango fan in Botswana, which are lowsinuosity to meandering bedload rivers, in which
banks are stabilized by peat levees. The contrasts in
river behavior when forests in upland, catchment
areas are destroyed by fi re or deforestation are
marked and well known. Runoff becomes more
flashy, the sediment load increases, and a tendency
may develop for debris flows to occur. The erosion of
steep-sided arroyos or box-canyons is typical of
flash-flood processes along basin margins and may
be an indicator of this type of sedimentation where
proximal fluvial deposits are preserved.
Temporal changes related to climatic causes may
be exemplified by several studies of rivers in the
United States. Nadler and Schumm (1981) and
Schumm (1985b) described examples of two rivers,
the South Platte and Arkansas, that evolved from
braided to meandering patterns within historic
times as a result of damming and irrigation that
resulted in lower discharge variability, lessening of
flood peaks, and stabilizing of banks by the growth
of vegetation. A climatic reduction in seasonality
would produce the same effect. Conversely, Schumm
and Lichty (1963) described the case of the Cimarron
River, Kansas, that changed from a suspension-load,
meandering morphology to a broad, shallow, bedload, braided morphology during a single major
flood in 1914. Channel widening continued until
1942, during a period ofbelow-normal precipitation,
which inhibited vegetation growth. However, local
factors unrelated to climate may produce the same
effects on channel style, as described in Sect. 8.1.
There may be major proximal-distal differences
in fluvial style between rivers in humid regions and
those in arid regions. In humid climates flow is perennial, and channel scale increases downstream as
discharge is added by tributaries, whereas in arid
climates there may be a loss of discharge downstream as a result of infi ltration and evaporation,
with a consequent reduction in channel scale. Rivers
may be perennial close to an upland, possibly alpine
source, but become ephemeral downstream toward
an arid basin center. For example, many of the
present-day rivers draining into the vast inland basins of western China show this characteristic. Sedimentologically, these differences may be recorded in
the scale of preserved bedforms and macroforms,
channel-fill cycles and flood cycles.
The terminal-fan model encapsulates many of the
fe atures of arid sandy systems, including the downstream decrease of channel scale, absence of channel
incision) and the lobate, distributary nature of the
river system (Fig. 8.71). this has now been well documented in several studies, as described in Sect. 3. By
contrast, the downstream increase in channel scale
433
structures fo rmed in the Tana River, Norway, by the
action of seasonal river ice. They included ice gouge
marks up to 1m wide and several tens of meters long,
eroded in the gravel and sand of the channel floor
and also on finer overbank sediments. A more subtle
effect included fl attening of current-formed clast
imbrication by pressure from stranded ice masses,
and scour holes arounded stranded ice blocks. Kettle
holes are depressions in river bed fo rmed by the melt
of buried ice masses. Some coarsening-upward
gravel-sand cycles in Quaternary outwash (Costello
and Walker 1972) have been interpreted as the result
of channel diversion into these depressions, which
deepen as melting takes place (Rust 1975). Some of
these criteria are illustrated in Fig. 12.3.
12.6 Sam! stones: The Significance
of Sand Body Architecture
and Sedimentary Structures
12.6.1 Fluvial Style in Sand-bed Rivers
Chaunel style in sand-bed rivers is controlled by
several factors, of which the amount and variability
of discharge, the nature of the sediment load, bank
composition, and bank stability, are among the more
important (Sect. 8.1). Some of these variables are
strongly affected by climate. Thus, humidity is the
main control on vegetation, which has an important
effect on discharge characteristics, sediment load,
and fl uvial style. In upland vegetated areas at the
present day, runoff following major rainfalls is rarely
catastrophic because the precipitation is absorbed
by soil and plants and released slowly. Also, sediment is stabilized by roots, and sediment yields are
relatively low. Therefore, as noted in the previous
section, mass-flow processes are less common in
humid climatic settings. Vegetated banks are highly
resistant to erosion, as demonstrated in an experiment by Smith (1976). This limits the supply of sediment to the river, and also reduces the bank
widening that is necessary for braiding to develop. In
vegetated areas, therefore, because of the effects on
discharge variability, bank stability, and sediment
supply, braiding is inhibited. Baker (1978a) quoted
examples in tropical Africa and South America
where dense bank vegetation inhibits braiding, even
where abundant coarse sediment is present in the
banks. Stanistreet et al. (1993) described the channels of the Okavango fan in Botswana, which are lowsinuosity to meandering bedload rivers, in which
banks are stabilized by peat levees. The contrasts in
river behavior when forests in upland, catchment
areas are destroyed by fi re or deforestation are
marked and well known. Runoff becomes more
flashy, the sediment load increases, and a tendency
may develop for debris flows to occur. The erosion of
steep-sided arroyos or box-canyons is typical of
flash-flood processes along basin margins and may
be an indicator of this type of sedimentation where
proximal fluvial deposits are preserved.
Temporal changes related to climatic causes may
be exemplified by several studies of rivers in the
United States. Nadler and Schumm (1981) and
Schumm (1985b) described examples of two rivers,
the South Platte and Arkansas, that evolved from
braided to meandering patterns within historic
times as a result of damming and irrigation that
resulted in lower discharge variability, lessening of
flood peaks, and stabilizing of banks by the growth
of vegetation. A climatic reduction in seasonality
would produce the same effect. Conversely, Schumm
and Lichty (1963) described the case of the Cimarron
River, Kansas, that changed from a suspension-load,
meandering morphology to a broad, shallow, bedload, braided morphology during a single major
flood in 1914. Channel widening continued until
1942, during a period ofbelow-normal precipitation,
which inhibited vegetation growth. However, local
factors unrelated to climate may produce the same
effects on channel style, as described in Sect. 8.1.
There may be major proximal-distal differences
in fluvial style between rivers in humid regions and
those in arid regions. In humid climates flow is perennial, and channel scale increases downstream as
discharge is added by tributaries, whereas in arid
climates there may be a loss of discharge downstream as a result of infi ltration and evaporation,
with a consequent reduction in channel scale. Rivers
may be perennial close to an upland, possibly alpine
source, but become ephemeral downstream toward
an arid basin center. For example, many of the
present-day rivers draining into the vast inland basins of western China show this characteristic. Sedimentologically, these differences may be recorded in
the scale of preserved bedforms and macroforms,
channel-fill cycles and flood cycles.
The terminal-fan model encapsulates many of the
fe atures of arid sandy systems, including the downstream decrease of channel scale, absence of channel
incision) and the lobate, distributary nature of the
river system (Fig. 8.71). this has now been well documented in several studies, as described in Sect. 3. By
contrast, the downstream increase in channel scale