Table 1. Descriptions of typical natural and altered disturbance regimes.

Natural

Altered


Connectivity within natural range
Magnitude Landslide dams may block migration to very small, e.g., a single tributary or large areas, e.g., most of the Fraser River Dams and culverts block small and large areas, and thousands of blockages are in place simultaneously.
Frequency Migration blockages are rare, < once every thousand years in most locations Not frequent, i.e., each blockage goes in once, but many areas blocked simultaneously
Duration Most landslide dams are temporary, lasting hours to days. Tens to hundreds of years to date
Predictability Locations are not very predictable. Once in place the blockage is “predictable”.

Sediment supply
Magnitude Magnitude of sediment supply varies spatially and temporally, driven by precipitation intensity, duration, and extent. Sediment supply generally increases due to land uses such as forestry, grazing, or cultivation.
Frequency Sediment supply is episodic. Some sediment enters a river system each year, but location, amount, timing, type, size, etc. vary from year to year. Land uses such as forestry tend to increase the frequency of sediment inputs to rivers.
Duration Duration of sediment supply generally mirrors storm durations. Little change from land use
Predictability Stochastic Stochastic, but more frequent

Hydrologic regime
Magnitude Spatial extent of storms and associated floods can be as small as a single watershed, or as large as an entire region, e.g., the 1964 storm, in which record floods occurred from northern California to southern Washington. Typically reduced due to dams absorbing the peak flows
Frequency Large floods or extreme low flows typically occur several times in a given year. Peak and low flows are typically reduced at the seasonal and annual time scale, whereas diurnal range may be increased at the daily time scale.
Duration Ranges between hours, days, and weeks Rivers regulated by dams typically increase the duration of peak flows.
Predictability Highly predictable on an annual scale, e.g., whether they will occur; less predictable at the monthly, weekly, or daily scale Greater predictability due to rivers being regulated by dams

Thermal regime
Magnitude Temperature extremes rarely exceed tolerances of most salmon populations. Temperature extremes commonly exceed tolerances of many salmon populations.
Frequency No exceedence in most years at most locations Several times a year in most locations
Duration Days to weeks Days to weeks
Predictability Predictable Predictable

Riparian vegetation
Magnitude Small streams: Fires periodically kill riparian forest patches.
Large rivers: River erosion periodically removes patches of floodplain forest.
Fires and erosion rarely kill riparian forest patches, but logging and land conversion kill a greater expanse of riparian forests.
Frequency Small streams: fire reset vegetation at return intervals of 50–400 yr
Large rivers: erosion reset forests at intervals of 1 to ~100s of yr
Logging: return interval of < 50 yr in most cases, riparian protections recently enacted in some areas
Land conversion: generally kills forests once
Duration Forests begin regeneration soon after disturbance. Logging: forests begin regeneration soon after disturbance.
Land conversion: often no regeneration, i.e., duration of decades to centuries
Predictability Fire: unpredictable
Erosion: predictable
Predictable

Nutrient regime
Magnitude Millions of salmon returning Few to no salmon returning in many areas
Frequency Annual Annual
Duration Salmon returning in most months of the year Salmon returning in most months, but shorter spawning runs for many populations
Predictability Predictable Predictable