20 - Stream Hydrology

65%

How much of Earth’s freshwater is in running water?

0.006%

How much freshwater do rivers deliver to the ocean each year?

30000km3/year

Where does ~55% of river discharge come from?

The 35 largest rivers on earth (Amazon biggest, Mackenzie biggest in Canada)

Why does the Nile discharge so little compared to its length?

It flows through an arid region, so less water enters the system.

What are the four key characteristics of running water?

• Water is almost always in motion

• Unidirectional flow (downstream)

• open ecosystem (constant input/output)

• High spatial and temporal heterogeneity

Why does very slow water become lake-like?

Because motion becomes wind-driven, not gravity drievn

What is a 1st order stream?

A single, unbranched headwater stream

What happens when two streams of the same order meet?

Their junction becomes a stream order of n+1

What happens when streams of different order meet?

The resulting stream keeps the higher order

Do lakes affect the stream order?

No, unless they are two streams of the same order

How does stream length change with order?

Stream length increases with order

Why are low-order streams more common?

Because high-order rivers rarely meet another river of equal order.

How does velocity change with depth?

It decreases exponentially due to friction with the bottom.

What factors affect discharge?

• Catchment size

• Climate

• Vegetation and soil type

• Slope

What is discharge?

The volume of water passing a point per unit of time

What two things are needed to calculate dishcarge?

Cross-sectional area x Flow velocity

How can flow velocity be measured?

• floating objects

• floating meters

• Constructed weirs

• Automated gauging stations

What is a the hyporheic zone?

The area beneath the streambed where groundwater and stream water mix

What is a gaining stream?

a stream that receieves groundwater input

What is a losing stream?

A stream that loses water to the surrounding soil/groundwater

What is a hydrograph?

a plot of a stream discharge over time

What is baseflow?

Flow maintained by groundwater and upstream water, not rainfall

What is storm flow?

The rise above baseflow caused by rainfall

What determines peak stromflow?

• rainfall intensity and duration

• catchment size and shape

• Soils and land cover

When is peak discharge for BC coastal streams>

Winter (warm,wet)

When is peak discharge for BC interior streams?

Spring (snowmelt/freshet)

What causes sediment transport?

water velocity —> shear stress on the bed

What is bed laod?

Sediment that rolls or bounces on the bottom

What is suspended load?

sediment carried in suspension in the water coloumn

What sediment is expected in low-order streams?

Boulders, cobble, gravel (more erosion)

What sediment is expected in high order streams?

Sand, silt, clay (more deposition)

What are meanders?

Stream curves that form on erodible floodplains

Where does erosion vs. deposition occur in meanders?

Outside bend: erosion

Inside bend: Deposistion

What is the thalweg?

the line of maximum flow velocity

What are riffles, pools and runs?

Riffles: shallow, fast turbulent

pools: deep, slow

Runs: fast, smooth, nonturbulent

pool-run-riffle