Cumulative change can occur in ecosystems such as watersheds. Effects within smaller watershed areas fall within many categories from sedimentation to dredging and organic debris distribution. Hydrologic effects and sedimentation from land disturbance are interrelated. Biological ramifications are dependant on watershed size, because there can be linear increases in fish species as stream order increases and there are numerous strong relationships among biological measures and stream size. Within watersheds, hydropower development can have cumulative effects.
Ecological effects vary with flow fluctuations and channel morphology.
Highly variable flows affect fish differently depending on the way a given species uses the habitat, and flow fluctuations reduce community complexity. These changes, however, are not always predictable, and mortality cannot be apportioned to sources of impact. Effects within watersheds can accumulate in large drainage basins. Within watersheds a fish species may or may not respond to habitat factors as described by laboratory experiments. Such response of fish populations makes quantitative predictions for small habitat modification impacts difficult.