RBC I Last week’s story on the Powell/Agency Park aquifers stemmed from a presentation by Dr. Mario Sullivan, instructor of science/oceanography at Colorado Northwestern Community College to the White River Alliance on March 15, 2022.
Former RBC resident and water resource engineer Collin Robinson wrote a detailed response, clarifying and/or correcting statements made in the article and March 15 presentation. You can read his response in its entirety here https://bit.ly/RobinsonResponse
Robinson challenges various statements made by Sullivan, and points out a geographical mix-up about the locations of Powell/Agency parks. “Agency Park runs from just across the river from town to the east. Powell Park is west of the Hogback,” he said. In our original story, the names were inadvertently swapped.
“CLASSIC” AQUIFERS
Robinson explained that there are two primary types of aquifers: non-confined/free aquifers and confined/artesian aquifers. Powell/Agency are “free aquifers” which generally means they’re shallow, alluvial aquifers that flow through “highly permeable, unconsolidated material like sand and gravel.” Additionally, free aquifers are often near surface streams, from which surface water percolates. These comments respond directly to statements that the local aquifers are not like “classic aquifers” because they do not have a confining bed or specific recharge point.
In contrast, “confined” or artesian aquifers are “capped by a tight non-permeable layer that holds water under pressure.” Robinson illustrated the differences by noting that free aquifers are like sponges, whereas confined aquifers are more like bagpipes.
AQUIFERS AS GROUNDWATER STORAGE
Citing streamflow gage and other hydrological data, Sullivan concluded that he didn’t see the aquifers as storage. According to Robinson, free aquifers actually have better storage potential than most confined aquifers, due to their sponge-like properties.
RETURN FLOWS
Return flow is when irrigation water applied to fields makes its way back into the river either as surface runoff, or as groundwater return flow after entering, and later exiting, aquifers.
Sullivan cited data modeling return flows on the Yampa River that indicated a “hydrological coupling” of both aquifers as part of a broader point about their high transmissivity. Dr. Bob Dorsett of Meeker said Sullivan’s understanding was “counterintuitive” since he and others understood the aquifers to act as a storage “buffer,” storing larger quantities of water temporarily and later transmitting it back to the surface.
Robinson’s explanation of the rate of “return flow” aligns more with Dorsett’s than Sullivan’s. Returning to the idea of a sponge, aquifers (particularly non-confined ones) absorb large quantities of water which will eventually return to the river.
“In some situations, water applied in June returns in July, in others, it begins to reach the river in December, and where distance is large, the lag may be several years,” Robinson wrote, adding, “in each case, the return is attenuated into a slow, steady, rate.”
Robison emphasized that in many cases, “surface return” is a more significant contributor to overall return flow than water flowing from groundwater/aquifer sources.
DRY RIVER STORIES
Sullivan also said “old stories” suggested that the river “did not flow year round in the middle-reach around the turn of the century, before irrigation ditches were put in place.”
Robinson said this claim was demonstrably false, citing senior water right records in the area. “Substantial irrigation diversion had already been occurring for over a decade before the turn of the last century,” he stated. Historical records of several local ditches listed in Robinson’s letter include the Powell Park Ditch (20cfs in 1880), Meeker Ditch (20cfs in 1883), Highland Ditch (45cfs in 1886), and Oak Ridge Park Ditch (25cfs in 1887) among others.
Diversions upstream from the middle reach prior to 1890 total at least 115 CFS, and Robinson says those diversions increased significantly in subsequent years. In fact he claimed that over-allocation is a more likely explanation of “channel dewatering.”
“Before development by American pioneers, the White River was a perennial stream through Agency Park,” Robinson concluded. “To the extent that the people of the valley continue to exercise self-restraint and teamwork during frugal times, the river remains, and will remain, a productive and beautiful perennial stream.”
By LUCAS TURNER | [email protected]om
