370
Geomorphic Patterns, Processes, and Perspectives in Aquatic Assessment
seasonal temperature regime affects types of precipitation and water storage and release (snow, permafrost, frozen soils, ice cover, glaciers). Climate
also strongly influences regional aquifer characteristics, such as whether stream relations with
groundwater are either influent or effluent.
Climatic classifications can be either genetic,
which reflect the actual climatic mechanisms or
processes, or parametric (Thornthwaite, 1948), using metric proxies for climate potential. The climatic metrics used in parametric climatic classification, such as evapotranspiration, average annual
precipitation, precipitation effectiveness, and intensity, influence biotic distribution and erosion as
well as hydrological variability. These metrics can
be derived from various map and information
sources, such as regional atlases of the United
States (e.g., Miller et aI., 1973).
To effectively interpret hydrologic variability,
we need to recognize the actual climatic processes
or mechanisms that affect hydrology. Different
types of atmospheric processes have different temporal-spatial scales of influence and can be scaled
by size, duration, and aerial coverage of precipitation (Orlanski, 1975; Hirschboeck, 1988). Hayden
(1988) presents a global atmospheric classification
for flood potential based on primary factors of temperature and water stored in the atmosphere and
secondarily as water stored in snow and ice. Other
climatic-hydrologic classifications include seasonal flood-climate regions for the United States
(Hirschboeck, 1991) and a flash-flood hazard map
for the United States (Beard, 1975). Climatic factors that influence flash floods are also classified
for the western United States (Maddox et aI., 1980).
The seasonal sequencing of these various climatic processes determines the hydrologic variation within and between regions (Hirschboeck,
1987). For instance, the variability of peak flows
in flood frequency curves between mountainous regions of the western United States reflects various
climatic mechanisms, such as snow, thunderstorms
and frontal systems (Pitlick, 1994). However,
Hirschboeck (1988) warns that stochastic-probabilistic hydrological analyses may mask the actual
climatic factors that cause variation and thus inhibit
adequate hydrologic explanation and prediction.
Thus hydrologic analysis needs to recognize that
populations of peak flows of similar magnitudefrequency can result from different types of climatic mechanisms and their seasonal sequencing
(Black, 1989; Hirschboeck, 1991). We also need to
consider different spatial scales in hydrological
analysis, because the hydrologic response to an atmospheric event is relative to the location within
and size of the watershed (Black, 1989), as well as
the size of the event.
25.4.2 Hydrological Characterization
Hydrologic processes perform a variety of ecosystem functions, such as formation of streambed and
floodplain fluvial features. Dynamic hydrologic
processes strongly influence riparian vegetation
composition and distribution (Gregory, 1991; Gurnell, 1995). Riparian composition is in part controlled by the frequency of inundation of different
fluvial surfaces across a valley (Hupp, 1990). In
tum, variability in valley characteristics constrains
hydrologic processes and consequent riparian composition (Hupp, 1990). Hydrologic dynamic character also affects distribution of aquatic organisms
relative to their life history characteristics (Poff and
Ward, 1989).
Most standard hydrological analyses are oriented
toward engineering or economic ends. However,
more ecologically oriented hydrologic metrics that
are being developed need to be considered for ecological assessments (e.g., Richter et al., 1997).
Stream hydrology can be characterized by a variety of methods that are well summarized in texts,
especially those by Leopold (1994), Gustard
(1992), and Rogers and Armbruster (1990). Two
basic metrics in watershed hydrology are the unit
hydro graph and hyetograph, which can be combined to compare streamflow relations to a single
precipitation event. These metrics compare the relationship between the time lag of a flow event to
a precipitation event relative to the magnitude of
the flow event, which is important in the interpretation of basin conditions. Such conditions include
antecedent soil moisture, the influence of basin
characteristics on water delivery processes, and alteration of the watershed by various natural and
land management processes (Leopold, 1991).
The flow record is a major tool in hydrological
interpretation and analysis, primarily as flood frequency or flow duration curves. However, the duration of the flow record severely limits interpretation, because the addition of very high or very low
flows changes the character of flood frequency and
flow duration curves to varying degrees, depending on the length of the flow record. For intensive
or long-term assessments, methods such as paleohydrological analyses can be used to extend the
record oflarge peak flows (Jarret, 1991). Flood frequency curves depict the relative magnitude and
frequency of peak flow events (the highest stage of
a flow event) and can be used to interpret hydrologic variability. The mean and variance of peak
Geomorphic Patterns, Processes, and Perspectives in Aquatic Assessment
seasonal temperature regime affects types of precipitation and water storage and release (snow, permafrost, frozen soils, ice cover, glaciers). Climate
also strongly influences regional aquifer characteristics, such as whether stream relations with
groundwater are either influent or effluent.
Climatic classifications can be either genetic,
which reflect the actual climatic mechanisms or
processes, or parametric (Thornthwaite, 1948), using metric proxies for climate potential. The climatic metrics used in parametric climatic classification, such as evapotranspiration, average annual
precipitation, precipitation effectiveness, and intensity, influence biotic distribution and erosion as
well as hydrological variability. These metrics can
be derived from various map and information
sources, such as regional atlases of the United
States (e.g., Miller et aI., 1973).
To effectively interpret hydrologic variability,
we need to recognize the actual climatic processes
or mechanisms that affect hydrology. Different
types of atmospheric processes have different temporal-spatial scales of influence and can be scaled
by size, duration, and aerial coverage of precipitation (Orlanski, 1975; Hirschboeck, 1988). Hayden
(1988) presents a global atmospheric classification
for flood potential based on primary factors of temperature and water stored in the atmosphere and
secondarily as water stored in snow and ice. Other
climatic-hydrologic classifications include seasonal flood-climate regions for the United States
(Hirschboeck, 1991) and a flash-flood hazard map
for the United States (Beard, 1975). Climatic factors that influence flash floods are also classified
for the western United States (Maddox et aI., 1980).
The seasonal sequencing of these various climatic processes determines the hydrologic variation within and between regions (Hirschboeck,
1987). For instance, the variability of peak flows
in flood frequency curves between mountainous regions of the western United States reflects various
climatic mechanisms, such as snow, thunderstorms
and frontal systems (Pitlick, 1994). However,
Hirschboeck (1988) warns that stochastic-probabilistic hydrological analyses may mask the actual
climatic factors that cause variation and thus inhibit
adequate hydrologic explanation and prediction.
Thus hydrologic analysis needs to recognize that
populations of peak flows of similar magnitudefrequency can result from different types of climatic mechanisms and their seasonal sequencing
(Black, 1989; Hirschboeck, 1991). We also need to
consider different spatial scales in hydrological
analysis, because the hydrologic response to an atmospheric event is relative to the location within
and size of the watershed (Black, 1989), as well as
the size of the event.
25.4.2 Hydrological Characterization
Hydrologic processes perform a variety of ecosystem functions, such as formation of streambed and
floodplain fluvial features. Dynamic hydrologic
processes strongly influence riparian vegetation
composition and distribution (Gregory, 1991; Gurnell, 1995). Riparian composition is in part controlled by the frequency of inundation of different
fluvial surfaces across a valley (Hupp, 1990). In
tum, variability in valley characteristics constrains
hydrologic processes and consequent riparian composition (Hupp, 1990). Hydrologic dynamic character also affects distribution of aquatic organisms
relative to their life history characteristics (Poff and
Ward, 1989).
Most standard hydrological analyses are oriented
toward engineering or economic ends. However,
more ecologically oriented hydrologic metrics that
are being developed need to be considered for ecological assessments (e.g., Richter et al., 1997).
Stream hydrology can be characterized by a variety of methods that are well summarized in texts,
especially those by Leopold (1994), Gustard
(1992), and Rogers and Armbruster (1990). Two
basic metrics in watershed hydrology are the unit
hydro graph and hyetograph, which can be combined to compare streamflow relations to a single
precipitation event. These metrics compare the relationship between the time lag of a flow event to
a precipitation event relative to the magnitude of
the flow event, which is important in the interpretation of basin conditions. Such conditions include
antecedent soil moisture, the influence of basin
characteristics on water delivery processes, and alteration of the watershed by various natural and
land management processes (Leopold, 1991).
The flow record is a major tool in hydrological
interpretation and analysis, primarily as flood frequency or flow duration curves. However, the duration of the flow record severely limits interpretation, because the addition of very high or very low
flows changes the character of flood frequency and
flow duration curves to varying degrees, depending on the length of the flow record. For intensive
or long-term assessments, methods such as paleohydrological analyses can be used to extend the
record oflarge peak flows (Jarret, 1991). Flood frequency curves depict the relative magnitude and
frequency of peak flow events (the highest stage of
a flow event) and can be used to interpret hydrologic variability. The mean and variance of peak