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You may think of your drinking water as coming from a source of water that is relatively consistent in its presentation; that is, it's always clear, clean and safe...right? But this is not always the case. Water utilities across the world face the sometimes troublesome task of trying to treat source water that has suddenly taken on different physical characteristics, such as cloudiness which is often called turbidity.

Rivers, lakes and oceans are not sterile bodies of water. Not only do they contain naturally occurring organisms and bacteria, they can be contaminated by outside sources. The most frequent sources of microbial contamination are polluted storm water runoff, sewage overflows, and boating wastes. Contamination in large bodies of water is often much higher during and immediately after rainstorms, because the rainwater picks up wastes and other pollutants as it runs off lawns, farms, streets and other ground sites and into the streams.

Every body of water has its own periods of change that is sometimes called a critical period where great care must be taken to maintain a satisfactory drinking water product.One example of a critical period for many waters is when flows of water suddenly rise. Worldwide, there are examples of streams naturally varying from stable to highly variable flows with subsequent changes in water quality. For example, sudden drops in flow, resulting from naturally occurring dry spells, or human-caused fill-up of reservoirs or water extraction, will leave water-saturated sediments exposed, thus increasing the risk for mud slides and turbid water.

Similarly, sudden increases in flow will also increase sediment discharge. For example, Old et al. (2005) describe natural jökulhlaups (glacier outburst floods) in the Skaftá River in Iceland in which discharge increased from 120 to 572 m3/s over 53.5 hours, and sediment concentration increased 5.4 times. Furthermore, sudden increases in flow in ice-covered rivers can cause ice jams, leading to flooding and potential nutrient enrichment of the river water.

Effects on water quality can also result when extreme flow events deviate from the norm in their timing. For example, it is well known from impounded rivers that long-term flooding, especially during the growing season, will destroy plant cover and erode soil, thus increasing the nutrient content of water but also its concentration of metals such as methyl mercury. This “damming effect,” which often leads to a rapid increase in, e.g., fish production, reaches a peak during the years following the erection of a dam, slowly disappearing thereafter. Gentle fertilization of such reservoirs has been suggested as a means of maintaining fish production (and fishing tourism) at a more sustainable level. Howitt et al. (2007) developed a model to predict unfavorable water quality associated with flooding in the Barmah-Millewa Forests on the River Murray, Australia and found that pooled floods and those in the warmest months of the year were substantially more likely to result in blackwater events (high levels of dissolved organic matter associated with low levels of dissolved oxygen) than floods in cooler times of the year and involving more water exchange between the river channel and the floodplain.

Water quality changes create critical periods because it is affected by many factors including weather, climate, rainfall, industrial and sewage discharges, and accidental spills. It is strongly impacted by runoff from rain events. This is easily observed by the eye during and after storms when streams and rivers are very cloudy and look brown. When it rains, the dirt, animal waste, and other contaminants that build up on the surface of the ground or pavement are washed off into the streams and rivers. Though there is more water in the streams and rivers during storms, there are more contaminants as well.

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