GEOS Chapter 18

subsurface flow

* Most storm water runs off in subsurface
*  Some flow occurs through soil matrix (porosity) -- (slow, small volume)
* Flow occurs preferentially in macropores (Root channels, animal burrows, structural cracks)
* Macropores are enlarged by erosion and form “pipes”
* Destruction of the macropores reduces infiltration, subsurface water conductivity, and promotes overland flow

saturated overland flow

* Water that soaks into the soil moves downslope in the soil
* As we move downslope the total volume of water passing through the soil increases
* If soil layer becomes fully saturated water springs on the surface as return flow
* Common near base of slopes with shallow soils

**stream**

* **long, narrow body of flowing water occupying a trench-like depression, or channel, and moving to lower levels under the influence of gravity.**
* **Stream discharge: the volume of water per unit time passing through a cross section of the stream at that location (cubic meters/second** to **m3/s)**

Stream Discharge Factors

**Climate:**Streams in wet regions have greater discharge than streams in arid regions.

**2. Streamorder:**First-orderstreamsareoften intermittent or have little discharge, even in the wettest of climates. Higher-order streams have more discharge.

**3. Season:**Peak flow periods depend on the timing of precipitation or snowmelt. Heavy rainfall can increase stream discharge to several hundred times its typical flow.

4\. Surface permeability:\*\*Permeable soil and rock allow water to infiltrate the ground rather than flow over it in a stream channel. Greater permeability reduces surface stream discharge.

Hydrograph

**shows the discharge of a stream in relation to time.**

Flood runoff

* component of flow which reaches the stream channel quickly (shaded green below). This is a surface runoff flow component.

Baseflow

* portion of the discharge of a stream contributed by groundwater flow. This is a sub-surface flow component

Lag time

* refers to the time taken for water to flow through the river channel from an upstream runoff area to a downstream flow measuring station
* Lag time depends on a number of factors
*  size of drainage basin feeding the stream
* whether there are large lakes in the system
* permeability of the underlying surface

Glaciers

Form in any area where:

* annual snow accumulation is greater than annual snow melt
* a large mass of ice that is formed from the \n accumulation of snow and flows slowly downslope
* Glaciers form where snow accumulates and is compressed by gravity to firn, then \n glacial ice.

Firn

* snow that has been compressed into granular ice and has an air content of roughly 50% of the total volume

Glacial ice

* ice that has an air content of less than 20% \n of the total volume

Glacier Ice Formation

* Formed from snow that does not melt in summer
* Accumulation of snow plus water causes increased weight which causes increased \n pressure

Snow transforms to ice:

* Pressure causes decreased air space which causes increased density, causing consolidation and recrystallization of snow into large crystals of ice (rough and granular)
* Lower layers turn to solid ice under weight of overlying ‘firn’ and snow
* Transformation is not just compaction, also snow metamorphis

Alpine glaciers

* glaciers found in mountainous areas
* There are almost 200k alpine glaciers in the world.
* Alpine glaciers occur on every continent except Australia

Ice sheets

* domed sheets of ice larger than 50000 \n km2 and can cover a significant portion of a continent
* Only Greenland and Antarctica have ice sheets

names of 4 main types of alpine glaciers

* cirque glacier
* valley glacier
* Piedmont glacier
* Ice cap

Piedmont glacier

* lobes of ice that form as a glacier flows onto a flat plain

cirque glacier

found at the head of a valley glacier and forms a bowl shaped depression

valley glacier 

occupies a mountain valley

Ice cap

large dome of ice that sits over a high mountain region

Outlet glacier

* a glacier that flows out of an ice sheet or ice cap through a constricted valley

Tidewater glacier

* outlet glaciers and valley glaciers that \n reach sea level and calve icebergs

Iceberg

* Large block of ice that breaks from toe \n of a glacier or an ice shelf and floats in the \n ocean or a lake

Ice shelf

* Portion of an ice sheet or outlet glacier that \n extends over the ocean

Zone of accumulation

* where ice gain from snowfall exceeds ice loss on the surface of a glacier

Zone of ablation

* where ice loss (melting, sublimation) \n exceeds ice accumulation on \n the surface of a glacier

Equilibrium line

* the transition between zone of accumulation and zone of ablation; also called firn line

Glacial advance

* occurs when a glacier has a positive mass \n balance, and the glacier toe advances forward and downslope

Glacial retreat

* occurs when a glacier has a negative mass \n balance; the glacier toe moves upslope in the process
* Glacier’s toe is continually advancing and retreating, but ice within a glacier is always flowing downslope even when toe is retreating upslope