Low Flow Database

Ecological Alterations

Along with the structural alterations when streamflow is restricted come changes in the habitat and the ecological communites (plants, animals, etc.) of riverine systems (aquatic and riparian). If streamflow is altered for long periods, brings wide swings in flows, or recurs more often than normal, profound and long-lasting changes to the ecological integrity of the natural communities can be devastating.

Habitat & the Physical Environment

Physically, a severe reduction in streamflow contributes to decreased oxygen levels, increased water temperatures, and concentrated pollutants. It also restricts navigation for both aquatic organisms and humans when pools become isolated from each other. Stagnant conditions increase the likelihood of algal blooms and contribute to the desiccation of streamside and wetland habitat and vegetation as the water’s edge recedes from the bank.

The amount and distribution of shallow, vegetated, deepwater, and other habitat types also changes when the river recedes from the streambank. Fish, freshwater mussels and other water-dependent organisms can suffocate when oxygen levels get too low, even if the stream still has some water in it. Fish species that prefer cold water, like trout and salmon, may not be able to survive when the cool, deep springs they depend on during warm summers become shallow and warm or stop flowing altogether.

Changes in the natural flow regime can also cause streambank instability. Banks that are normally covered with water become unstable when they are exposed to desiccation, wind, or freeze and thaw cycles. When water returns in the form of surface runoff or increased flow, these banks have a higher likelihood of eroding and causing sedimentation problems downstream.

Vegetation

Extreme low flows and rapidly fluctuating streamflow conditions can cause changes in riparian and aquatic vegetation. Many riparian species are adapted to the naturally occurring cycles of wet and dry periods and cannot reproduce successfully if conditions are not right for germination or seed disperal, for example. Additionally, altered flow regimes often favor invasive exotic species that can survive under less variable and lower flow conditions. In many rivers with low flow problems, purple loosestrife is one common invader that displaces native vegetation but does not serve the same ecological functions.

Flowing Water Adaptations

Many riverine aquatic organisms depend on continuously flowing water for some part of their life cycle. For species that depend on flowing water, a year with no flow means a year with no reproduction. For short- lived species, several years of extreme conditions can seriously reduce the size of the population and its ability to repopulate the river when sufficient streamflows return.

Trout lay their eggs in a nest (redd) over clean gravel and even fan the redd with their tails to keep it free of sediment and well-oxygenated. Without enough flow to help them keep sediment from settling on the eggs and enough oxygen in the water, these eggs will die!

Some freshwater mussels release their eggs (glochidia) in a fish-shaped lure that they suspend in the water column. Fish then eat the lure and the eggs attach to the fish’s gills to complete the next part of their life cycle. Without enough flow in the river to support the lure in the water column, the fish won’t eat the lure and the eggs can’t attach to the fish’s gills!

Net-spinning caddisfly larvae spin a web to collect their food from the water column. Without enough flow to support their nets these caddisfly larvae have no source of food!

Community Structure

A lack of flowing water wreaks havoc with the entire aquatic community. When river dependent species can’t successfully reproduce or are driven out of a river because of poor habitat conditions, the fish and macroinvertebrate assemblages shift from being dominated by river-dependent species to being dominated by pond-like species. In the Ipswich River, which has serious summer low-flow problems, 95% of the fish are habitat generalists, or pond-like species, as opposed to only 55% generalists in the target, or ideal, Ipswich River fish community . Aquatic macroinvertebrate assemblages at sites with little to no flow are frequently dominated by midge larvae and have a reduced number of sensitive species such as mayflies, stoneflies, and caddisflies.

Estuaries

Seventy to ninety percent of the world's commercially important marine species depend on estuaries and their delicate balance of fresh and salt waters at some point during their lives. Forty percent of all continental runoff, or all the water that falls over land as precipitation and runs into rivers and ultimately estuaries, is intercepted by man-made reservoirs along the way. This means that there is currently seven times more water stored in rivers than there was before the era of large dam building. Controlling the amount of fresh water that reaches estuaries has a huge impact on estuarine habitats and the organisms that depend on them.

Parker River estuary, Newbury. (Photo by Tom Warhol.)

Salinity, or "saltiness", is often a major limiting factor for organisms that spend all or part of their lives in estuaries. Too much or too little salt can mean the difference between life and death for many organisms, particularly during their early life stages. Changes in river flow can alter the salinity in estuarine habitats and effectively isolate organisms from their preferred habitats such as sea grass beds, rocky intertidal zones or salt marshes. River flow also controls the depth and temperature of many estuarine habitats.

Shifting salinty gradients can have different effects on estuarine organisms depending on where they live. Benthic organisms (those that live on the bottom of the estuary), such as clams, mud snails or the eggs of many species, generally move too slowly to adjust to changes in salinity and are often hit hardest by altered freshwater flows to estuaries. Organisms that move more readily will generally follow their preferred range of salinity around the estuary, but may have a smaller area of adequate habitat available to meet their needs for food or reproduction.

Over a period of several years, changes in the amount of freshwater flowing from a river into its estuary can cause shifts in the structure of the estuarine community. Much like the changes in community structure that occur in rivers with low stream flow problems, species that depend on freshwater or low salinity habitats in estuaries will tend to have less successful reproduction and higher mortality as river flows decline and will gradually be replaced by species that prefer saltier water. This can often give non-native generalist species an advantage over native species that are adapted to the historic conditions in a particular estuary.

Islands of clamshells in the Colorado River Delta (above) illustrate the 90% decline in clam abundance due to the lack of fresh water flow from the Colorado River.

In addition to the decline in overall clam abundance, Mulinia coloradoensis (left), a clam that depends on the flow of fresh water, is being replaced by Chione cortezi (right), which prefers the saltier conditions that exist without freshwater inflow from the Colorado River. (Photos taken from the Centro de Estudios de Almejas Muertas at the University of Arizona).