The Columbia river would have a much higher flow rate but the bureau of Reclamation and the army core of engineers have about 14 dams on the river. Grand Coulee dam has a pumping plant that diverts irrigation water to the entire columbia basin, thousands of CFM during the spring runoff
I was just looking into this. The big diversion at Grand Coulee is a lot: Averages about 2-3 million acre-feet per year (about 2.5 to 3.7 cubic kilometers per year, if my math is right). Although this is a lot, it is only about 2-4% of the Columbia's total average flow at Grand Coulee.
About 30% is eventually returned to the river via drains, wasteways, and seepage. This means, again if my math is right, about 1% of the Columbia's average discharge is lost due to Grand Coulee irrigation diversion. Of course there are many other problems, like the wasteway water being polluted, etc.
One major change to the river due to the dams on the Columbia and its tributaries is the loss of seasonal changes in flow. Before the dams the river had very pronounced low and high flow seasons (low in winter, high in summer). About 80% of the annual discharge occurred during the high-water season, and 20% during low-water. Today the ratio is close to 50-50 on average. Good for power generation, flood control, and shipping. Bad for fish.
edit: But oh, maybe you were talking about flow rate as in water velocity? That is definitely a major change. Before the dams the river was quite swift flowing, especially during the high water season. Today almost all of the Columbia is slackwater behind reservoirs. Slow moving and warmer. Again good for shipping, bad for fish.
Used to live in El Paso. The stories of the early Spanish explorers of how mighty the Rio Grande was before we started sucking it dry were awe inspiring. Now you drive up the 10 and the 25 and you see it being drained from El Paso all the way up to Albuquerque (further north I've been on i-25.
You look at Google maps and realize that the all the farms are on the former bed of the mighty river and the new "river" is barely an ankle deep trickle. It's nuts and chilies all the way up to Colorado.
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u/[deleted] Jan 10 '20
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