Geology Reference
In-Depth Information
Table 3.4 Classification of valley sediments
Type of deposit
Description
Channel deposits
Transitory channel deposits
Resting bed-load. Part may be preserved in more durable channel fills or lateral
accretions
Lag deposits
Sequestrations of larger or heavier particles. Persist longer than transitory
channel deposits
Channel fills
Sediment accumulated in abandoned or aggrading channel segments. Range
from coarse bed-load to fine-grained oxbow lake deposits
Channel margin deposits
Lateral accretion deposits
Point bars and marginal bars preserved by channel shifting and added to the
overbank floodplain
Overbank floodplain deposits
Vertical accretion deposits
Fine-grained sediment deposited from the load suspended in overbank
flood-water. Includes natural levees and backswamp deposits
Splays
Local accumulations of bed-load materials spread from channel on to bordering
floodplains
Valley margin deposits
Colluvium
Deposits derived mainly from unconcentrated slope wash and soil creep on
valley sides bordering floodplains
Mass movement deposits
Debris from earthflow, debris avalanches, and landslides, commonly intermixed
with marginal colluvium. Mudflows normally follow channels but may spill
over the channel bank
Source:
Adapted from Benedict
et al.
(1971)
and rougher channels upstream promoting erosion; and
shallower gradients, larger hydraulic radii, and smoother
channels downstream promoting deposition. In addi-
tion, flat, low-lying land bordering a stream that forms
a suitable platform for deposition is more common at
downstream sites.
Alluviation may be studied by calculating
sediment
budgets
for alluvial or valley storage in a drainage
basin. The change in storage during a time inter-
val is the difference between the sediment gains and
the sediment losses. Where gains exceed losses, stor-
age increases with a resulting
aggradation
of chan-
nels or floodplains or both. Where losses exceed gains,
channels and floodplains are eroded (
degraded
). It is
feasible that gains counterbalance losses to produce
a steady state. This condition is surprisingly rare,
however. Usually, valley storage and fluxes conform
to one of four common patterns under natural con-
ditions (Trimble 1995): a quasi-steady-state typical of
humid regions, vertical accretion of channels and aggra-
dation of floodplains, valley trenching (arroyo cutting),
episodic gains and losses in mountain and arid streams
(Figure 3.13).
GLACIAL PROCESSES
Ice, snow, and frost are solid forms of water. Each is a
powerful geomorphic agent. It is convenient to discuss
