Page 93 - Fundamentals of Geomorphology

76 INTRODUCING LANDFORMS AND LANDSCAPES

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 a steady state. This condition is surprisingly rare,
shallower gradients, larger hydraulic radii, and smoother however. Usually, valley storage and fluxes conform
channels downstream promoting deposition. In addi- to one of four common patterns under natural con-
tion, flat, low-lying land bordering a stream that forms ditions (Trimble 1995): a quasi-steady-state typical of
a suitable platform for deposition is more common at humid regions, vertical accretion of channels and aggra-
downstream sites. dation of floodplains, valley trenching (arroyo cutting),
Alluviation may be studied by calculating sediment episodic gains and losses in mountain and arid streams
budgets for alluvial or valley storage in a drainage (Figure 3.13).
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- GLACIAL PROCESSES
nels or floodplains or both. Where losses exceed gains,
channels and floodplains are eroded (degraded). It is Ice, snow, and frost are solid forms of water. Each is a
feasible that gains counterbalance losses to produce powerful geomorphic agent. It is convenient to discuss