In the summer of 2007 a rainstorm caused a large sediment inflow from Seven Castles Creek into the Fryingpan River and downstream into the Roaring Fork River. There were concerns expressed by the natural resource agencies and public regarding potential impacts to macroinvertebrates and fish habitat within the river. It was suggested that a flushing flow may be needed to remove the sediment and restore the function of the system. The objective of this study was to collect sediment distribution from freeze cores, quantify macroinvertebrates and qualitatively assess habitat to assist the agencies in determining whether a flushing flow was required prior to the natural high flow.
Three sites were selected for this study which were: just downstream of Taylor Creek, just downstream of Seven Castles Creek, and within the City of Basalt, all within the Fryingpan River. Taylor Creek and Basalt sites have previous data from the 2003 and supplemental studies that were used for comparison to the conditions after the August rainstorm.
Sediment distributions were assessed by using freeze core sampling. Two freeze cores were collected at each site and analyzed for sediment distributions. Macroinvertebrates were assessed with the same methods as previously applied which included three Hess samples collected at each location and standard metrics of diversity, evenness, density and biomass analyzed for each site. Habitat was visually assessed with the majority of the work being concentrated near Seven Castles Creek.
The freeze core analysis showed that sediment distribution at all sites ranged from fine to coarse material with no distinct differences between any of the sites. The freeze core sample at Taylor Creek showed a rounded cobble substrate (Figure ES-1). The Seven Castles site was more angular, likely due to the inflow of new sediment from Seven Castles Creek but the distribution was similar to other sites (Figure ES-2). Distribution by size show that the samples collected in 2007 are very comparable to the samples collected previously with the spawning study (Figures ES-3 and ES-4, Table ES-1).
Macroinvertebrate sampling resulted in the highest diversity and evenness at Taylor Creek and Basalt. The Seven Castles Creek site had the lower values for all metrics. Taylor Creek and Basalt were in the range that would be considered good and were very similar to the previous studies. The Seven Castles Creek site was rated as fair but within the same range as those seen in the previous study mainly at the site collected just downstream from the reservoir.
Habitat was assessed at Seven Castles Creek and at each of the other sites by visual observation. The large sediment delta at Seven Castles Creek has moved the Fryingpan River channel away from its previous channel location and into the riparian area. A new channel is being cut through the riparian vegetation and there are multiple channels for approximately 100 yards downstream from the confluence. This likely will result in a loss of vegetation within the riparian area, in particular the large cottonwood stand is now Fryingpan River Seven Castles Creek Study Page ii Miller Ecological Consultants, Inc. March 6, 2008 being inundated. Those mature cottonwoods may die due to the change in channel and the flooding that is occurring in that area.
The results of the study show that the sediment distribution is similar to that observed prior to the event and that at all sites the sediment distribution is not extremely different. The fine material that is evident at some locations seems to be very locally distributed and limited to those local areas of small eddies along the bank or along the stream margin where it was deposited during the high water. Within the channel the substrate itself seems to be in the same condition or close to the same condition as prior to the event.
The macroinvertebrate community is lower in density than previously observed with the lowest density occurring near Seven Castles. However, the density, diversity and evenness show that the downstream condition near Basalt is not substantially different than the condition at Taylor Creek upstream of the inflow. The location at Seven Castle closest to the inflow is different from Taylor Creek but appears to be recovering with a variety of macroinvertebrates present at that site.
At this time it is recommended that no additional flushing flow occurs prior to the annual spring runoff and that any additional water, if available, should be applied at the natural peak. This will avoid impacts to invertebrates that would occur from runoff events that would be outside of the natural cycle. The riparian area with new channels is one location that may require some mechanical assistance in moving the channel back into its original location. It is likely that this channel migration occurred in the past, and especially prior to Reudi Reservoir. Historically, spring flows would have caused the channel to migrate back into its previous location and flushed out sediment from those areas. Since the peak flows are considerably less than historically experienced, it may be beneficial to mechanically move some of the material, especially the large rock at the confluence, to allow high flows to move downstream in the old channel location. In addition, the exposed fine sediment at the confluence with Seven Castles Creek could be a potential site for revegetation with young cottonwoods to establish a cottonwood gallery to replace the mature cottonwood gallery.
- The sediment inflow did have an immediate effect on the confluence area of Seven Castles Creek
- A new Fryingpan River channel was formed.
- The average of the Taylor Creek samples was only slightly coarser than the samples collected at Seven Castles Creek and Basalt.
- Seven Castles Creek had one of the coarsest samples collected of all six samples collected.
- Macroinvertebrate results show reduced macroinvertebrates at all locations compared with the previous studies.
- The most impacted area appears to be the immediate vicinity of Seven Castles Creek. Fryingpan River Seven Castles Creek Study Page iii Miller Ecological Consultants, Inc. March 6, 2008
- The impact appears to decrease with increased distance downstream from Seven Castles Creek.
- In the lower Fryingpan, the biota appear to be less impacted and in the same condition as the Taylor Creek reference site.
- A flushing flow outside of the normal runoff period is not recommended.
- Any flushing flow should coincide with the normal high flow.
- It is not likely to scour the large boulders deposited in the Fryingpan River with the normal high flow.
- This material will likely remain in place for many years unless it is mechanically removed.
- Mechanical removal may assist in placing the channel in its previous location and reduce impacts to the riparian area that is now flooded.
- An alternative would be to allow natural stabilization to occur in the new channel and use the new delta area for cottonwood and willow planting.