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Hydrology people talk about gaining and losing streams. This doesn’t mean what it sounds like—as usual.
The short version is that a stream gains if it collects water as it crosses the landscape. A stream is a losing stream if it emerges smaller than when it entered the area. Some, like the Humbolt River, never return from the desert.
How much water a river gains or sheds depends on a whole lot of variables, even before you toss human water use into the mix. The kind of climate, the kind of bed material, the season, all those shape the stream. A river with a solid rock bed, that flows through a place where it rains a lot all year around, will gain water. A river with a sandy bed and a water-table below river level, in a place that inclines toward dryness, with highly seasonal rainfall, will be a losing stream in that area, much of the time. But that can change on short notice, which is called flash flooding (or the seasonal rise in some places.) If water can seep out of the river more than other streams and rain put into the stream, it is a losing river. It “loses” water as it crosses the landscape, departing with less than it started with.
Once humans start tapping the ground-water, gaining streams can become losing streams. The Canadian used to flow relatively steadily most years, and springs from the Ogallala Aquifer provided much of the winter flow. Because the water table was level or even slightly above the river, water flowed in through springs and seeps, as well as rain and snow. When center-pivot irrigation spread through the Canadian River watershed in Texas, that lowered the water table. The river stopped gaining water and began losing it, at least during irrigation season. After the late 1970s and the early 1980s, the water table dropped far enough that even winter wasn’t enough to restore flow, and the water in the river sank down to the water table. The Canadian had become a losing stream instead of gaining or even steady.
Different sections of streams are called “reaches.” One reach may be gaining, then the next loses water, then a later section gains. This is true of many rivers that flow through the central United States, although the Missouri and Mississippi start gaining sooner than their tributaries do. The Mississippi especially benefits from being well east of the 20″ rainfall line. The Missouri used to go almost dry most years in its upper stretches, until humans put in dams and changed the flow schedule in order to make the river navigable for more of the year. The Canadian often went dry in summer, and flooded in late May-June. And occasionally flooded seriously in the fall, thanks to hurricanes and other low-pressure systems that parked and poured water into the upper reaches of the stream.
This year, 2019, has seen an atmospheric water pump park over the Gulf Coast, feeding moisture up into the Great and High Plains. This, combined with a series of cool fronts, has led to lots of large, slow-moving, heavy rain storms, or multiple storms that cover the same area. The results are a lot like what hit Iowa, and southern Minnesota in the 1980s and 1990s, and the flooding is all across the nightly news. People downstream are bracing, as steady streams gain rapidly, then take all that down to the Gulf.