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My Water Quality: Aquatic Ecosystem Health - Streams, Rivers & Lakes

California Streams, Rivers and Lakes

Why is hydrologic connectivity important?

Hydrologic connectivity describes the ability of water to move through a watershed. Connectivity can help maintain sediment balance and sustain riparian vegetation corridors. Connectivity also creates linkages between aquatic and terrestrial habitats and allows wetland-dependent species to move between habitats as needed for feeding or reproduction. Water flows allow aquatic life, sediment, and nutrients to move through the stream ecosystem.

There are three types of connectivity that are important to stream functioning. Water can flow along a stream’s length and through the network of channels in a watershed. Even streams that become dry during the summer may maintain connectivity to the stream network through underground flow. Streams also connect to the surrounding landscape: drainage from adjacent land areas feeds stream flow, and water from a stream can spread across its floodplain or into tidal marshes. Connection to the floodplain can provide a range of functions, including slowing water, trapping sediment and contaminants, and creating backwater habitat. Finally, streams are connected to groundwater. Water in streams can replenish or drain groundwater sources. Each of these types of connectivity affects stream form and function and is beneficial to overall stream health.

Many human modifications alter natural patterns of connectivity, making stream systems more fragile. Dams and other physical barriers can interrupt connectivity along the length of a stream. Levees along stream banks and channel deepening due to aggressive erosion (also known as downcutting, or incision), can limit the connectivity between a stream and its floodplain. Streambeds laden with fine sediment can limit connectivity between a stream and the underlying groundwater. Modifications can also cause problems by increasing the connectivity between a stream and the landscape. Pavement and storm drains speed flow of often-polluted runoff from urbanized areas into streams, creating short periods of extreme flow that contribute to stream bank erosion, stream downcutting (incision), and flooding.

How do we measure hydrologic connectivity?

To assess hydrologic connectivity, managers can measure the quantity and fluctuation of flow throughout a year and along the length of a stream. Scientists also measure the width-to-depth ratio or the cross section configuration of streams as an indicator of how connected a stream is to its adjacent floodplain.