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Texas Living Waters Project / Environmental Flows / What are environmental flows?

What are environmental flows?

“Environmental Flows” is a scientific term used to describe the quantity, quality and timing of water that are necessary to sustain the fish and wildlife of a river, wetland or coastal zone.

Environmental flows are made up of both “instream flows”— flows in a river or stream—and “freshwater inflows”—the flows of fresh water that make it down the river or stream and into an estuary system. An environmental flow “regime” is a prescription of different amounts of flows that varies by both the time of the year and by hydrologic conditions. A flow regime for a river or stream includes:

  • Subsistence flows: those very low flows seen only during times of drought;
  • Pulse flows: short-term events brought on by heavy rainfall producing a surge of water in the river and/or into the bay system; and
  • Base flows: those flow volumes that fall between subsistence flows and pulse flows and that occur most of the time; generally there are several different levels of base flows.

Rivers and estuaries are naturally dynamic ecosystems that rely on periodic changes in flow to maintain their health and productivity. Keeping our rivers, streams, estuaries and wetlands healthy is important for many species of wildlife and for the state’s economy. Texas has seven major coastal estuary systems, formed where fresh water from rivers mixes with the saltier water of the Gulf of Mexico. This mixture of salt and fresh water is vital for many marine species-including fish, shrimp, oysters and crabs-and fuels a $2 billion+ coastal recreational and commercial fishing economy.

Healthy rivers, wetlands, and estuaries are also important for many of the 600+ species of birds found in the state. Bird species that migrate through Texas and the numerous year-round residents rely on these habitats, and the fish, shellfish and plant life that they sustain, for their food source.

Freshwater fish in Texas’ rivers need sufficient amounts of clean water to survive. Many species of fish also need higher flows at certain times of year so they can move to their spawning grounds and reproduce. Coastal estuaries and bays are considered “nurseries of the sea” because they provide just the right balance of salinity, nutrients, and temperatures that allow for young fish and shellfish to mature.

In addition to animal species, many plant species also depend on adequate environmental flows for their life cycles. Many wetland habitats, such as bottomland hardwood forests, depend on periodic flow out of the banks of the rivers and streams for seed dispersal and a natural culling of invasive species.

Ash Creek at Woodward Ranch, South of Alpine Photo courtesy of Charles Kruvand