Deltas: Source of most of the coastal sediment
Site of much human settlement: proximity to the sea, abundant food supply (fish etc.), and
fertile ground. Modern people have also realized that they are the source of many petroleum
deposits.
Delta development
More sediment than can be reworked by coastal processes
Change in sediment transport (loss of unidirectional current in the ocean/lake)
River system significance and Plate Tectonic Setting
Drainage basin area
Topography
Continental Margin width (active vs. passive margin)
Climate and Glaciers-reduce size of continental shelf
Mississippi R. delta
Ohio, Missouri, Miss. Rivers
Young part of delta is 5000-6000 years old
16 recognizable lobes of the delta
50% of Louisiana has been formed by the river in the last 6000 years
Environments
Transitional between terrestrial and marine
Gradational boundaries between environments
DISTRIBUTARY CHANNELS
Major environments are:
Delta Plain-mainly river influenced (terrestrial)
Delta Front-mainly subtidal (marine)
Prodelta (subtidal)
Delta Plain-coastal extension of the river system
Channels and their deposits onto the floodplain, channel cut off and oxbow lakes,
crevasse splays, levees, etc.
Flooding brings sediment (vertical accretion) to the delta plain as the water slows down
and deposits its sediment load
Channel Migration/Distributaries, produce lateral accretion
Delta Front
Seaward edge of the delta plain-generally subtidal region of delta
Suspended and bedload sedimentation as water velocity drops at end of distributary
channel (distributary mouth bar may develop-causing the channel to bifurcate)
Suspended load material may move farther offshore
Deltas divided into
River Dominated
Tide Dominated
Wave Dominated
River Dominated:
Widely separated river mouth bars and distributary channels. Fluvial (river) processes
dominate-channel/levee/crevasse splay. Interdistributary channel areas alternately fill in and
erode as the channel switches.
Tide Dominated
Tidal channel-style distributary channels, often funnel shaped and longitudinal bars
reworked by strong tides. Often full of ill-defined maze of tidal current ridges. Flood and ebb
tide produces sedimentary structures indicative of two directional currents
Wave dominated deltas
Wave processes redistribute most of the sediment supplied to the delta front. Regular
beach shorelines, sand spits, and Abandoned beach ridges are common.