This week I field validated my hyporheic confinement hypothesis for a site I have been working on.
I had been out mapping wetlands and characterizing a system of ditches and stream-like features. Lucky for me, a chinook was blowing: soil thawed sufficiently to be observed each afternoon. With not-normal effects on vegetation and soil chemistry from seasonal saturation by a nearby irrigation ditch, I suspect these two particular wetlands would delineate smaller, jurisdictionally speaking, come the growing season in March. But I don't know for certain. The combination of river and irrigation induced hydrology can be confounding.
Many of the stream-like ditches used at the site to accommodate irrigation water and return flows were dry. For the ones that had flow I had a devil of a time getting into them safely to measure their cross sectional profile. Prior to my client's purchase for a residential/golf course project, the property was used to run a cow/calf operation. Much of the lower ditch (15 - 30 feet across) has 20 plus inches of anaerobic mud and manure, a sure recipe for disaster for the hip wader approach. The occasional gravel bar saved me from having to pontoon for my data.
The ditches are running with mostly hyporheic/phreatic Yakima River water. I say mostly, because some snowmelt was running in a small ditch onto the site from the upland terrace onto the floodplain. The Yakima is 1000 feet away and was running near bank-full. The ditches are running a few inches below the ordinary high water scour line, and I feel certain the two hydrologies are connected.
The concept that hyporheic/phreatic hydrology can reach this far is a challenge for most folks, including my fellow project team members. How can river groundwater hydrology be feeding it when the ditch is higher than the river? The answer lies in subsurface gravel filled channels. Rivers lose and gain the same water repeatedly. In losing reaches, water drops out of the bottom into permeable gravel filled channels. Where these channels are covered with less permeable material, confinement can result in a considerable buildup of gravitational head. Where the gravel channel reaches to the margin of the floodplain, confined water can upwell at considerable distance from the river, and can be confused with irrigation derived groundwater.
In the Yakima Valley, with its 500,000 irrigated acres and its network of leaky canals, irrigation induced seasonal wetlands are common. In the floodplain, upwelling hyporheic/phreatic river water can be masked by irrigation induced hydrology, but only while the canals are full, or recently so. During this January visit, long after irrigation diversions have ceased, there was no mistaking the dominant river-induced hydrology at the site. Especially telling was the water level in an existing stream-like ditch compared with the newly constructed closed ditch intended become its replacement. Closed at the upper end, the upwelling river derived groundwater flowing in the new ditch was higher by 14 inches than the water flowing in the adjacent, topographically upgradient, closer-to-canal, older, connected, irrigation district return flow structure. 14 inches is also consistent with seepage on the bank of the older ditch structure. In the photo these are separated by only 60 feet.
These 2 ditches provide the strongest validation I've seen in the 20 years I have been observing and puzzling over hyporheic confinement and upwelling.