37
interest (White and Walker 1997). Historical data can be problematic because of
unmeasured factors that confound the interpretation of observed historical changes.
An analysis designed to relate changes in channel geometry to precipitation and
stream flow in a forested catchment, for example, might misinterpret a change in
channel geometry associated with a widespread blowdown that recruited substantial
large wood to the channel network. Inferences based on reference sites can be problematic because of the difficulty in finding a close match in all relevant parameters
(White and Walker 1997). Use of either historical data or reference sites can be
constrained by limited information (Fig. 2.11), but comparison to relatively natural
river corridors remains the most widely used method of assessing the effects of
human alterations on river ecosystems.
Despite the difficulties of characterizing natural range of variability and reference conditions, attention to four primary aspects of river corridors as part of river
management can enhance and sustain river health. The following sections explore
each aspect in more detail.
2.5.1 Connectivity
Connectivity describes movements of energy, material, and organisms between
components of a system, which are typically spatially defined areas. Structural connectivity (Wainwright et al. 2011) results from physical contiguity (e.g., water flows
from a hillslope directly onto a floodplain). Functional connectivity results from
Fig. 2.11 Schematic illustration of natural range of variability. The black circle indicates the
width/depth ratio of a river segment on an altered tributary. The blue line represents variations
through time in the width/depth ratio of pristine tributary A: the natural range of variability is
indicated by the dashed blue lines defining the upper and lower limits of the width/depth ratio for
this site. The brown lines represent similar information for pristine tributary B. If the altered tributary is compared only to site A, it lies within the natural range of variability. Comparison to site B
suggests that the altered tributary is outside of the natural range of variability
2.5 The 4Cs of River Health
interest (White and Walker 1997). Historical data can be problematic because of
unmeasured factors that confound the interpretation of observed historical changes.
An analysis designed to relate changes in channel geometry to precipitation and
stream flow in a forested catchment, for example, might misinterpret a change in
channel geometry associated with a widespread blowdown that recruited substantial
large wood to the channel network. Inferences based on reference sites can be problematic because of the difficulty in finding a close match in all relevant parameters
(White and Walker 1997). Use of either historical data or reference sites can be
constrained by limited information (Fig. 2.11), but comparison to relatively natural
river corridors remains the most widely used method of assessing the effects of
human alterations on river ecosystems.
Despite the difficulties of characterizing natural range of variability and reference conditions, attention to four primary aspects of river corridors as part of river
management can enhance and sustain river health. The following sections explore
each aspect in more detail.
2.5.1 Connectivity
Connectivity describes movements of energy, material, and organisms between
components of a system, which are typically spatially defined areas. Structural connectivity (Wainwright et al. 2011) results from physical contiguity (e.g., water flows
from a hillslope directly onto a floodplain). Functional connectivity results from
Fig. 2.11 Schematic illustration of natural range of variability. The black circle indicates the
width/depth ratio of a river segment on an altered tributary. The blue line represents variations
through time in the width/depth ratio of pristine tributary A: the natural range of variability is
indicated by the dashed blue lines defining the upper and lower limits of the width/depth ratio for
this site. The brown lines represent similar information for pristine tributary B. If the altered tributary is compared only to site A, it lies within the natural range of variability. Comparison to site B
suggests that the altered tributary is outside of the natural range of variability
2.5 The 4Cs of River Health