r&b: bedrock rivers

the five bedrock river characteristics

the five bedrock river characteristics

fixed channel; high transport capacity; low storage; input = output; strong interaction between streams and hillslopes

what does it mean to have low storage for sediment?

implies all sediment inputs are balanced by downstream transportation

hill-slope channel coupling

when sediment falling from a hillside goes directly down into a river

what sets the pace for landscape lowering in areas where landslides are common?

river incision (if river can't incise bed while also carrying deposited material, then valleys will fill and hillsides lower)

bedrock channels

floored by bedrock, lacking alluvial cover; indicative of high transport

where do waterfalls occur?

where cutting-down occurs (typically where two different types of rock that have two different weathering rates meet)

are waterfalls forever?

no (only exist as long as barrier does)

do waterfalls stay in one place?

no, very transient (move backwards relative to flow)

hanging valleys

catalyst for a glacial waterfall; occur when glacier incises into rock that is higher than floor of main valley

three ways a stream erodes its channel

hydraulic action, solution, abrasion

hydraulic action

occurs when pressure and turbulence of water flow physically moves rock grains; can wedge open preexisting fractures; particularly effective at steep gradients

solution

chemical weathering of bedrock (most prevalent in limestone); can produce large amount of mobile sediment; flowing water increases dissolution rate, deepens channel

abrasion

grinding of bed via friction; impacts rock fragments and sediment grains

base level

defined in many ways, but for now: limiting level below which a stream cannot erode

four types of natural dams

landslide sediments, glacial deposits, glacial ice, and lava flows

what do dams result in?

local base levels (lakes and such)

global base level

ocean

what happens in a stream when base level goes down?

erosion

what happens in a stream when base level goes up?

sediment accumulation

steady-state channels

occur when rate of erosion and rate of uplift roughly balance

topography

relief, slope, and drainage basin morphology

mountain range growth

when rates of uplift are greater than rates of erosion; equals increase in slope and relief

mountain range decay

when rates of erosion are greater than rates of uplift; equals decrease in slope and relief

are the mountain range growth/decay states sustainable?

no; will constantly give way to other depending on erosional rates and tectonic activity

five young valley characteristics

V-shape; rapids and waterfalls; no floodplain; drainage divides are broad and flat; valley actively deepening

five younger mature valley characteristics

V-shape; floodplain present; sand and gravel bars; sharp drainage divides; maximum relief is approached/approaching

maximum relief

when valleys stop deepening

three older mature valley characteristics

wider valley bottom; narrow floodplain; rounded divides; diminished relief

two old valley characteristics

peneplain surface (nearly flat), only erosional resistant rocks remain; river meanders across wide, flat floodplain

what four human activities have lead to increased sediment yield?

clearing of forests, cultivation of land, damming of streams, construction of cities, etc

how much sediment reaches the ocean as a result of anthropogenic damming?

has cut amount of sediment reaching the ocean by half