Human Impacts on Stream Hydrology

Measuring Discharge and Hydrographs

Measurement of discharge is crucial in understanding stream hydrology.
Hydrographs are graphical representations of stream flow over time, illustrating different flow regimes in a water system.
Flow regimes influence aquatic biota and are affected by various factors, including human activities.

Human activities have multiple impacts on stream hydrology.
This discussion focuses on three key impacts:
- Dams
- Roads and road networks
- Deforestation

Two dominant impacts of dams on flow regimes include:
- Reduction of peak flows: Dams reduce the high flow events typically seen in natural systems.
- Increase in base flows: Dams can increase water flow during dry seasons by releasing stored water.

Dams implemented for flood control have significantly altered the hydrograph.
- Construction of Lookout Point Dam and Falls Creek Dam: Both built to mitigate flooding issues for nearby residents.
- Resulted in:
- Topped off peak flow events, thus decreasing winter peak flows.
- Increased base flow during summer, aiding agricultural irrigation.

Changes in stream flow patterns negatively impact local biota:
- Black cottonwood (Populus trichocarpa): Adapted to flood conditions, trees rely on high flows to carry seeds downstream for establishment.
- Dams lower peak flows, which leads to shallow rooting systems and decreased stability.
- Cutthroat trout (Oncorhynchus clarkii): Migration and reproductive success are affected by altered flows.
- Changes in spawning flow conditions lead to lower survival rates for eggs and fry, along with altered migration cues.

Strategies are being considered to recreate historic flow patterns even if the magnitude of flooding cannot be mimicked.
Encouragement for understanding dam impacts on aquatic life and the efficacy of restoration efforts.

Roads modify landscape hydrology through:
- Impedance of infiltration: Roads have impermeable surfaces that prevent water absorption.
- Interception of groundwater flows: Roads alter the flow of groundwater towards streams, resulting in faster runoff.

Studies demonstrate how roads lead to increased runoff:
- Stream hydrographs illustrate quicker response times and higher magnitude of peak discharges post-road construction.
- Roads promote flashier systems, which may harm aquatic habitats by altering flow.

Roads significantly change hydrology, leading to more frequent and intense flooding conditions and altering stream ecosystems' health.

Forest management, particularly logging, can lead to notable changes in stream flow:
- Clear-cutting forests can result in rapid increases in discharge.
- These changes can be immediate, but lead to long-term hydrological alterations.

Studies indicate:
- Maintenance of riparian buffers does not fully insulate streams from watershed-wide hydrological impacts.
- Initial increase in stream discharge following logging can later be followed by a significant decrease over decades as systems mature.

Before-After-Control-Impact study design shows:
- Clear-cut areas show increased discharge immediately post-logging, but this declines significantly over time.
- Long-term studies reveal that
- Areas that are logged multiple times exhibit diminished stream flows in later years compared to their initial states.
- Forest management practices need to consider both short-term and long-term impacts on hydrology.

The impacts of human activities such as dam construction, road development, and deforestation have profound effects on stream hydrology and aquatic ecosystems.
Understanding these effects is crucial for biodiversity conservation and effective water management strategies.