Meacham Creek Geomorphic – Hyporheic Study III

In summer 2011, the Confederated Tribes of the Umatilla Indian Reservation (CTUIR) completed a large channel restoration project along a one-mile (1.6 km) reach of Meacham Creek, an important salmon-spawning tributary to the Umatilla River, Oregon (Figure 1, Final Report). This restoration effort included channel realignment (re-meandering), addition of large woody debris to the site with the specific objective of increasing hyporheic exchange, and extensive riparian re-vegetation. The overarching objectives of the project were to:

This project continued ongoing post-project, hypothesis-driven research that evaluated whether objective 3 (Increase hydraulic connectivity including hyporheic and river water exchange and improve the three dimensional hydrologic mosaic.) of the restoration activity was met. This research combined a variety of field and numeric modeling techniques to create a complete picture of the residence time distribution for hyporheic water at the restoration site for both pre- and post- restoration conditions and documented the effects of channel re-alignment on hyporheic exchange, hyporheic flow path lengths, residence time, and ultimately, channel temperature.

Project methods included three facets: A water temperature analysis and two analyses of geomorphic change within the restoration reach – channel pattern changes and topographic differencing between 2004 and 2016. Analysis of water temperature data from 2005-2015 shows that there is a significant decrease in water temperatures in the reach below the 2011 Meacham restoration project, although the mechanisms that underlie this change are somewhat ambiguous. Additionally, we assessed channel changes in the three target reaches, using the available aerial photography and calculated complexity metrics (River Complexity Index [RCI], Brown 2002) that show, predictably, a high degree of change throughout each reach. Finally, we estimated the cut and fill volumes between 2004 and 2016 using DEM differencing of high resolution topographic datasets (James et al. 2012).

To compare the changes across 11 years of summer water temperatures, before and after the 2011 restoration effort, we used a paired t-test. We calculated mean values from the pre and post project years (pre 2005-2010 and post 2011-2015) from the years that contained both RCI and water temperature data to populate the t-test. The results of the t-test included:
• Increased RCI values in the Line Creek Reach are significantly related to the declines in water temperature beginning in 2011
• Both the Restoration and Control Reaches had increased RCI values following the stream restoration project of 2011
• Water temperatures subtly declined in the Control Reach (decreased by an avg. .1C) and rose in the Restoration Reach (increased by an avg. .5C)

Other efforts associated with this work were instructive, but did not result in crisp results that were comparable to changes in annual summer water temperatures. Although the DEM differencing maps and cut/fill calculations provided an interesting perspective on the integrated changes between 2004 and 2016. We anticipate using these data to further explore the geomorphic changes imparted in the lower Restoration and Line Creek Reaches to build our understanding of the conditions that underpin reduced summer water temperatures in this area of Meacham Creek. This effort analyzed 11 years of water temperature data at loggers that bracketed three reaches – below, at and above, the 2011 restoration project on Meacham Creek. We find a progressively reduced summer water temperatures below the 2011 Meacham Creek restoration project, as measured at the downstream boundary of the Line Creek reach. Using a simple t-test with variances, we show that the negative water temperature changes are highly correlated with increases in the River Complexity Index (RCI). This result is consistent with previous studies on the Umati

The channel reconfiguration associated with the 2011 Meacham Creek Restoration Project (Fig. 1, Final Report) was designed to re-establish historic channel planform patterns, move the channel back atop floodplain gravels, recreate normative channel migration patterns and provide a more natural water temperature regime in the study (2011 restoration) reach. To document these changes, we instrumented the area above, within and below the reach (the restoration reach) with surface water temperature loggers. The project had a number of effects on hyporheic hydrology and surface water temperature. The expected benefits to native salmonids include the areas of more natural water temperature regimes that contribute to all freshwater life stages.

As intended, the more complex channel form of the restored channel was associated with an increase in the residence time distribution of hyporheic water, enhancing the amount of hyporheic water with residence times less than one month. These streambed-, bar-, and meander-scale flow paths are associated with buffering and lagging of daily and seasonal water temperature cycles that can improve water temperature regimes for salmonids (Arrigoni et al. 2008, Amerson et al. In preparation). Overall, more heat was exchanged between the channel and aquifer post-restoration, as is indicated by the increased rate of warming of the restoration reach, in contrast to the control reach (upstream) and the Line Creek reach (downstream). In essence, the additional heat entrained in the aquifer over the summer was removed from surface water as it passed through the restored reach. This heat will be released by the aquifer and returned to the channel as autumn channel water temperatures decline, the warm water exfiltrates from the aquifer, and cooler water infiltrates the aquifer.

Also, the realignment of the channel back to the center of the floodplain and atop alluvial sediments increased the water surface elevation of the channel, and the associated water table elevation across the restoration site. Preliminary results from this project suggest that the increase in saturated sediment volume not only increases the heat storage capacity of the aquifer, but also forces an increase in the rate of hyporheic exchange to maintain the elevated water table.