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2,880

2,880

3,240,000

Of the approximately 5,200,000 total river kilometers in the contiguous United States, nearly half are first order, and the total for first- to third-order combined is just over 85%. Examples of large rivers include the Allegheny (seventh order), the Gila (eighth order), the Columbia (ninth order), and the Mississippi (tenth order).

Of the approximately 5,200,000 total river kilometers in the contiguous United States, nearly half are first order, and the total for first- to third-order combined is just over 85%. Examples of large rivers include the Allegheny (seventh order), the Gila (eighth order), the Columbia (ninth order), and the Mississippi (tenth order).

not uncommon - they include over 20,000 river channels in the contiguous United States. About 250 US rivers are of order seven and higher, and only a handful are great rivers (Table 1.2). Of these, only the Mississippi ranks among the 15 largest rivers of the world based on the annual volume of its discharge.

Each stream or river drains an area of land that is proportional to its size. This area is its drainage basin, and includes the topographically determined region that contributes all the water that passes through the stream. Although it is common to define drainage area for the entire river system from origin to mouth, one can also determine the drainage area for individual tributaries. Clearly, then, just as the stream network forms a hierarchy of low-order streams nested within higher-order rivers, their drainages likewise are nested hierarchically. As with rivers and streams, there are overlapping terms in wide use. Drainage basin and river basin tend to be applied to large units, whereas smaller units are referred to

TABLE 1.2 The 25 largest rivers of North America, ranked by their virgin (corrected for withdrawals) discharge. (From Benke and Cushing 2005.)

River name Discharge Basin area

TABLE 1.2 The 25 largest rivers of North America, ranked by their virgin (corrected for withdrawals) discharge. (From Benke and Cushing 2005.)

River name Discharge Basin area

1 Mississippi

18,400

3,270,000

2 St. Lawrence

12,600

1,600,000

3 Mackenzie

9,020

1,743,058

4 Ohio

8,733

529,000

5 Columbia

7,730

724,025

6 Yukon

6,340

839,200

7 Fraser

3,972

234,000

8 Upper Mississippi

3,576

489,510

9 Slave (Mackenzie Basin)

3,437

606,000

10 Usumacinta

2,687

112,550

11 Nelson

2,480

1,072,300

12 Liard (Mackenzie Basin)

2,446

277,000

13 Koksoak (Quebec)

2,420

133,400

14 Tennessee (Ohio Basin)

2,000

105,870

15 Missouri

1,956

1,371,017

16 Ottawa (St. Lawrence Basin)

1,948

146,334

17 Mobile

1,914

111,369

18 Kuskokwim

1,900

124,319

19 Churchill (Labrador)

1,861

93,415

20 Copper

1,785

63,196

21 Skeena

1,760

54,400

22 La Grande (Quebec)

1,720

96,866

23 Stikine

1,587

51,592

24 Saguenay (St. Lawrence Basin)

1,535

85,500

25 Susitna

1,427

51,800

Note that drainage area alone is a rough predictor of volume of flow.

Note that drainage area alone is a rough predictor of volume of flow.

as watersheds and subwatersheds, or catchments and subcatchments.

Worldwide, rivers are classified by their hierarchic position within river basins and within regions. The US Geological Survey Hydrologic Unit Cataloging (HUC) system (Seaber et al. 1987) catalogs watersheds at different geographical scales based on 1:100,000 mapping (Table 1.3). It first divides the United States into 21 major regions that contain either the drainage area of a single river or the combined drainage area of a series of rivers. Smaller units are nested within regions. An eight-digit HUC is standard, and typically corresponds to a drainage area in the order of 103 —104 km2. Further subdivisions exist or are under development, resulting in 11- or 14-digit watersheds.

The view of river systems as a hierarchically arranged, nested series of units provides a powerful organizing framework in which to examine the patterns and processes of fluvial ecosystems (Frissell et al. 1986) (Figure 1.3). The largest scale is the river basin and drainage network.

TABLE 1.3 Hydrologic Unit Cataloguing illustrated for a specific watershed, Mill Creek (04173500), which enters the Huron River upstream of Ann Arbor, Michigan.

Code Name

Number in the

Average

segment

United States

area (km2)

04 Water resource

0 0

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