|Monthly streamflow of the Dismal River (data from the USGS)|
|Monthly streamflow for the Animas River (again from the USGS)|
Like breath flowing in and out, or blood coursing through a vein, most rivers have some sort of regular cycle of up and down. In the case of the second chart (the Animas River) it is the springtime snowmelt that causes the repeating pattern. Even snow-free rivers have cycles, perhaps because of a summer monsoon or a rainy season. Even if the rainfall is the same every month, higher evaporation in the summer versus winter can cause seasonal cycles. In places without strong cycles, there's still usually some sense of variability, perhaps because one year was wetter than another.
The Dismal River, however, is different in that it has nearly no cycle or variability. Rain or shine, summer or winter, Tuesday or Saturday, the flow is nearly always around 225 cubic feet/6.4 cubic meters per second. It's a healthy amount of flow, certainly not a trickle, but it is just relentless.
|The River tries not to take its name personally. The steep far bank shows where the grass is stripped away and the ground underneath is very sandy.|
|The Dismal River Streamgage in 2012 (photo courtesy of USGS Nebraska Water Science Center)|
If today's rainfall doesn't influence the runoff, then what does? To some extent, atmospheric pressure! When the air pressure is low, more water comes out of the ground.
|Manually measuring streamflow (photo courtesy of USGS Nebraska Water Science Center)|
Why is the river going up since the 1980s? I asked USGS hydrologist Ron Zelt and he noticed precipitation in the area has gone up about 11% and snow has gone up 15% between 1960 and 2000. He felt the rise is partly caused by the sometimes lengthy (e.g., decades long) periods when warmer (or cooler) sea-surface temperatures in the North Pacific Ocean contribute to differences in air pressure and atmospheric circulation patterns. This affects the weather in central Nebraska, and the signal is even greater in the North and West. Well levels were rising relatively rapidly from the 1980s until the start of the 2000s.
Then, about a decade ago (i.e., in 1999), a long period with a generally warmer-than-average North Pacific Ocean ended, and with it, a period of increased rain and snowfall in Nebraska gave way to drought during 2000-06. There is about a ~9 year lag as the rainfall works its way through the soils from the surface to about 100 feet down. It is these lower levels that feed the Dismal River's flow. Since the rainfall has not been what it used to be since 2000, well levels have done a U-turn and started to drop. 2002 was the second driest year in the last 50 and other years were almost as poor. Depending on what the rainfall does in the future, it might be a few years before the river, as well, eventually starts drifting down again to more dismal levels.
|Dried out grasslands in 2002 (photo courtesy of USGS Nebraska Water Science Center)|
In the rest of this blog, I've tried to highlight a few "hydrologic oddities" (follow the link to the "best of" page for a running list). These are unique, strange, weird rivers that just don't fit the standard "leaky bucket model" of how hydrologists think watersheds should behave. It is important to remember that the extremes include rivers that are un-predictable but some that are also unusually predictable, such as the Dismal.
In that spirit, here's a trivia question - when it comes to months-ahead river forecasting (like the kind highlighted here), what part of the Western US do you think is the most predictable, i.e. where are the forecasts are the most accurate? If you think you have an idea, leave a comment below...