Flows and Transport in Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 87

• The gage site is far enough upstream from the conluence with another stream or from a

tidal effect to avoid any variable inluence from other streams or one that the tide may have
on the stage at the gage site.

• A satisfactory reach for measuring the discharge at all stages is available within reasonable

proximity of the gage site. (It is not necessary to measure low and high lows at the same
stream cross section.)

• The site is readily accessible for easy installation and operation of the gaging station.

An ideal site is rare. Also, the controls and cross-section geometry may change in response to
high-low events or other factors, affecting the rating curve, as illustrated in Figure 4.5. Therefore,
the rating curve must be updated periodically.
The accuracy of indirect estimates depends on the accuracy of the low measurement, the
accuracy of the rating curve, the completeness and accuracy of the gage-height record, and the
degree to which changes occur, as depicted in Figure 4.5. The accuracies of the U.S. Geological
Survey (USGS) discharge records for individual days are typically considered to be on the
order of 90%-95% (errors of 5%-10%; Hirsch and Costa 2004), but can be much less in some
locations.

4.3.2 M eaSureMent of M orpHoMetry
Once the location is identiied, a ield survey is typically done to measure the elevation of the chan-
nel bottom from some vertical datum at a series of horizontal locations across the channel, also
georeferenced. General survey details can be found in Benson and Dalrymple (1984) and Kennedy
(1990). Since the site may need to be rechecked periodically, a permanent benchmark should also
be established to tie the cross-section and longitudinal proiles to an elevation control for future
comparison. Channel surveys are also a critical component of river management, and the measure-
ments taken will vary with the intended use of these data. For example, for low measurements,
only data at the low measurement location may be of importance. However, ield surveys for river
morphology or hydraulic studies require multiple cross sections to establish the river proile, over
some reach of hydraulic interest (such as in a lood zone determination) or management interest
(such as multiple meander wavelengths).

Feet

10

9

8

7

6

Before After

5

4

3

2

1

FIGURE 4.5 An example of change cross-section geometry. (From USGS, Available at http://ga2.er.usgs.
gov/peachtree/streamlow.cfm.)