Streams Geology 115

Hydrologic cycle

Surface water Precipitation falls and becomes overland flow Overland flow organizes into streams “Stream” is a generic term covering brooks, creeks, rivers, really any flowing surface water which is not the ocean

A stream’s drainage is its watershed Continental Divide All of the streams in a drainage are called the stream’s network

Stream profile Base level

Stream gradient % gradient = (elevation gain) ÷ s(horizontal distance) Water velocity is fast Water velocity is slow % gradient = (elevation gain) ÷ s(horizontal distance)

Streams erode and deposit to fit the ideal profile The base level may also be affected by rising or falling sea levels

Waterfalls alter the stream profile The underlying rock is quite erosion-resistant

As the stream tries to achieve the ideal stream profile, the waterfall retreats as the rock is eroded Plunge pool develops

Niagara Falls State Park, New York

The fall line –

A stream’s discharge is the volume of water that flows past a given point per time for that stream When a river floods, its discharge increases, mostly because the stream’s width increases and not so much its depth.

Parts of a stream Tributaries Headwaters

More vocabulary words about streams

Braided streams are characteristic of rivers near their headwaters Denali NP, Alaska

Meandering streams are characteristic near their outlet Vicksburg NMP, Mississippi

Floodplain and levee Levee, enhanced with a revetment Former stream channel Floodplain

Levee, enhanced with a revetment Floodplain Cut bank Point bar

Meanders both erode and deposit Cut bank Point bar

Meanders migrate

The material deposited by a stream is called alluvium, which tend to be rich in nutrients, especially metals, which makes for a good soil

An alluvial fan is a feature of a stream that emerges from a mountain gorge onto a plain (Death Valley NP, California)

Different types of stream mouths - delta Distributaries Mississippi River Delta NHA, Louisiana

Different types of stream mouths - estuary Estuaries are major coastal wetlands that provide not only wildlife habitat but also remove metals and organic toxins from streams prior to the ocean Oregon Dunes NRA, Reedsport, OR

Different types of stream mouths – drowned river mouth Chesapeake Bay

Stream evolve over time – for instance, meander migration causes valleys to widen Oxbow lake

Antecedent streams – the stream was there first, and, by gum, it’s going to keep to its path whatever uplift or erosion happens Fold mountains Peneplain mountains Fault block mountains

The Potomac River at Harpers Ferry NHP (WV, VA, MD) These ridges are made of a resistant metamorphic rock called quartzite

Consequent streams follow the topography (they don’t cut through) In this example from Virginia, Beaverdam Creek was an antecedent stream that incised through a ridge of resistant rock; it was captured by Shenandoah River (a consequent stream) in an act of stream piracy.

A water gap is where the river cuts through resistant rock (Harpers Ferry NHP) A wind gap is where the river once cut through resistant rock but has been captured by another river and only a low spot in the hills remains

Cumberland Gap NHP (TN, KY, VA) was an important wind gap for Native Americans and settlers through the Appalachian Mountains Note that fog uses it, too!