Monitoring Restoration Effectiveness on the Lower Rio Blanco River
J.D. Kurz1 and D.L. Rosgen2
Overview The Rio Blanco River is located in Southwestern Colorado near Pagosa Springs, Colorado. The watershed encompasses a 170 square mile (mi2) area that ranges from 13,000 feet at the headwaters along the Continental Divide to 6,400 feet at the confluence with the San Juan River. Since 1971, the U.S. Bureau of Reclamation has operated a major trans-basin diversion called the San Juan/Chama Project. This diversion transmits approximately 70 percent of the historic annual water yield from the Rio Blanco to the Rio Grande Basin. The diversion not only removes a substantial volume of water from the channel throughout the year, but it also decreases the magnitude and duration, and alters the timing of the channel-maintaining flow (bankfull discharge). Since the dimension, pattern, and profile of rivers are directly related to the bankfull discharge, a natural decrease in the bankfull discharge should result in a series of adjustments creating a smaller channel, more appropriately sized for the altered flow regime. Unfortunately, the channel below the diversion has not adjusted to the altered flow regime mainly because the magnitude, duration, and timing are insufficient to reshape the channel material, which consists mainly of cobble and large gravel. Sediment supply from unregulated tributaries creates excess sediment deposition into the regulated main stem. The resulting wide and shallow channel has negatively impacted the channel stability, water quality, and aquatic habitat of the lower Rio Blanco. In November 1999, Wildland Hydrology completed a stream restoration demonstration project on 1.1 miles of river. The goal of the restoration project was to construct a channel to match the altered flow regime in order to restore the physical and biological function of the river. The width/depth ratio was decreased to improve sediment transport capacity and reduce water temperature. Rock vanes were installed to reduce bank erosion and improve aquatic habitat by creating a diversity of bed features. A monitoring program was initiated following construction to document channel adjustments and restoration effectiveness with respect to the physical objectives. The restored reach was re-surveyed in 2001 following a spring runoff season comparable to the pre-diversion flow regime. In addition, vertical velocity profiles were measured at both restored and un- restored reaches to compare the distribution of energy in the near-bank region.
Hydrology Pre-Diversion Hydrology The Rio Blanco has a snowmelt-dominated hydrograph. Snowmelt runoff generally peaks during the first week of June and the flow during the summer commonly drops below 20 cfs. Historically, the major floods have been the result of intense thunderstorm activity in the late summer and fall months. Analysis of the peak flow records from the USGS stream gaging station number 09343000, which operated upstream of the point of diversion between 1935 and 1971, indicates that the 1.5-year momentary maximum peak discharge was 726 cubic feet per second (cfs) (approximate bankfull discharge) prior to the San Juan/Chama Project
1. J.D. Kurz, Hydrologist, Wildland Hydrology Consultants, Pagosa Springs, CO 2. Dave Rosgen, P.H., Hydrologist, Wildland Hydrology Consultants, Pagosa Springs, CO josh/c:mydoc/rivers/Colorado/lblanco/monitoring/2002/monitoringrestorationeffectiveness Post Diversion Hydrology The San Juan/Chama Project can divert up to 520 cfs from the Rio Blanco River into a series of tunnels that eventually reach the Rio Grand Basin. The U.S. Bureau of Reclamation is required to bypass at least 15 cfs during December through mid-February, 20 cfs mid-February through April, 40 cfs during May, and 20 cfs during June through November. Flow is bypassed during unusually wet years when the New Mexico reservoirs are full or the when tunnel capacity is exceeded. Flow was bypassed during the flood of record, which occurred on August 24, 1992 when thunderstorms produced a peak flow of 3120 cfs. The USGS operated a gaging station (09343300) downstream of the diversion from 1971 until 1998, at which point Colorado Division of Water Resources began maintaining the gage. Analysis of the peak flow records from this station (1971-1998) indicates that the 1.5-year momentary maximum peak discharge is approximately 463 cfs for the post-diversion flow regime (subtracting 100 cfs from Leche Creek, a 8mi2 tributary between gage stations 09343000 and 09343300). The diversion removes 263 cfs or 36 percent of the bankfull discharge. The operational hydrology of the Rio Blanco is unlike most regulated rivers in that the bankfull discharge is decreased, but the floods are bypassed.
Consequences Channel Stability Decreasing the magnitude and duration of the bankfull discharge has decreased the capacity and competence of the river to move the larger fraction of materials being delivered from the watershed. The decrease in competence and capacity is compounded by insufficient shear stress that is associated with the wide, shallow channel that is remnant of the previous flow regime. Not only is the magnitude and duration of the bankfull discharge decreased, but the timing is also altered, which leads to in-channel deposition as unregulated, high sediment supply tributaries deliver sediment into the Rio Blanco. Since more energy is required to re-initiate motion of a particle than to keep an entrained particle moving, the sediment delivered from the unregulated tributaries is deposited in the Rio Blanco, which leads to aggradation and lateral migration. During floods, both shear stress and stream power are low and deposited gravel bars re-direct velocity vectors into less resistant streambanks. The net result is an increase in the width/depth ratio as bars grow and banks erode in order to maintain channel capacity. High width/depth ratio channels have less shear stress and more bed resistance, which increase the flood stage relative to a channel that has the same cross sectional area but a lower width/depth ratio. High width/depth ratio channels are often associated with poor physical water quality and aquatic habitat.
Water Quality The high width/depth ratio (W/d) channel leads to higher water temperatures and increased evaporation from the large surface width. High water temperature is a major problem when the streamflow drops below 20 cfs during the summer and water temperatures and dissolved oxygen approach the lethal limit of trout populations. At 20 cfs, many sections of the Lower Blanco are 40 feet wide with a mean depth of 0.5 feet (W/d=80). During the winter months limitations for fish occur due to the lack of thermal refugia from the shallow mean depth and absence of deep pools.
Aquatic Habitat The existing high width/depth ratio channel has a poorly defined thalweg. During low flow the stream lacks the depth necessary to create good instream habitat, which is necessary to support a viable fishery. Natural reproduction is inhibited by the accumulation of fine sediment in spawning gravels during baseflow.
Restoration History The enabling legislation for the San Juan/Project required that the Bureau of Reclamation operate the diversion is such a way that the fishery of the Rio Blanco would not be degraded. A U.S. Forest Service Report (Huchinson, 1990) concluded that the altered flow regime has resulted in a wide, shallow stream with inadequate habitat, cover, and water temperatures to support a coldwater fishery. The Colorado Water Conservation Board, Rio Blanco Homeowner Association, San Juan Water Conservation, and several federal agencies have been working together to obtain funding to improve the stream habitat of the lower Rio Blanco River. Funding was secured for a demonstration project through Section 319 of the EPA Clean Water Act. According to the Colorado Division of Wildlife (WIP, 1998), the water quality issues are increased water temperatures, decreased levels of dissolved oxygen, and the accumulation of fine sediments. During the fall of 1999, Wildland Hydrology and Elk River Construction completed a 1.1-mile restoration project on the lower Rio Blanco.
Objectives The goal of the restoration project was to construct a channel to match the altered flow regime in order to restore the physical and biological function of the river. More specific objectives were to improve sediment transport and flood capacity, reduce bank erosion, improve the physical water quality by decreasing water temperature and increasing dissolved oxygen, and improve fish habitat and spawning gravel.
Design The designed channel must maintain a deep, narrow, low flow channel and also be able to accommodate the large floods that bypass the diversion. A small, low width/depth ratio channel meanders within the existing over-wide C3 and F3 stream types. Material excavated from the low flow channel was used to construct floodplains and point bars that extend to the remnant floodplain. The low flow channel was constructed in the shade cast by the alders and willows whenever possible to assist in maintaining low water temperature. J-Hook Vanes and Cross Vanes were constructed with large boulders that were excavated off-site. The J-Hook Vanes reduce bank erosion and maintain deep pools downstream of each structure. Cross Vanes provide grade control, reduce bank erosion and maintain deep pools below each structure. A Cross Vane was used to protect a bridge within the reach and several other Cross Vanes were used to raise the elevation of the channel bed through a degraded reach in order to re-attach the floodplain as well as to raise the low flow water level for stream-side water pumps. In addition, streamflow is aerated as it cascades over the rock structures into deep pools below.
Monitoring Channel Stability Twenty permanent cross sections were installed to monitor channel change and/or to study the effects of the rock vanes on the distribution of channel energy in restored versus unrestored reaches. In addition, a longitudinal profile was surveyed through the entire demonstration reach immediately following completion of the project. The cross sections and longitudinal profile were resurveyed in September 2001 to document channel change following an above average runoff year in which 1040 cfs was recorded on May 14th. This large flow event (twice bankfull) provided an excellent test of restoration stability and effectiveness. Replicate plots are not included in this paper due to size limitations, but they will be shown during the oral presentation and will be available through the Wildland Hydrology office. Substantial redistribution of channel material occurred throughout the restored reach during the 2001 spring runoff. Many of the pools below J-Hooks and Cross Vanes scoured to bedrock, allowing these pools to maintain depths of up to 4 feet at low flow. The elevation of the riffles increased slightly or remained unchanged. The width of the low flow channel increased slightly, but a narrow, deep, low flow channel was maintained. Very little bank erosion occurred throughout the restored reach. The lack of bank erosion can be attributed to the bank protection provided by J-Hooks and Cross Vanes, which redirect velocity vectors away from streambanks. The Cross Vane that was installed upstream of the problematic bridge has reduced the stress against the bridge footers.
Water Quality and Aquatic Habitat The Colorado Division of Wildlife is monitoring water temperature, dissolved oxygen, siltation, and biological parameters. Five monitoring stations measured temperature during the three years prior to construction and there is currently 2 years of post-construction temperature data. Water quality and fish/macroinvertabrate data is not yet available. The aquatic habitat has improved as holding water and overhead cover have been created.
Summary Although flow depletion is an unfortunate reality for the lower Rio Blanco River, channel restoration is a viable option to improve channel stability, physical water quality, and aquatic habitat. The demonstration restoration reach performed well during the first high water test. The narrow low flow channel was maintained and deep pools still exist below J-Hooks and Cross Vanes. The restoration has improved the biotic potential of the dewatered stream and has improved the recreational and aesthetic values as well. Children enjoy swimming in the deep pools and one landowner said she loves to be able to hear the river again, even at low flow.
Bibliography Huchinson, Corey Sue. 1990. Survey of the Rio Blanco Basin, 1990. USDA Forest Service Report, San Juan National Forest, Durango, Colorado. Water Information Program. 1998. 33 Responses to Questions from the October 5, 1998 Lower Blanco Habitat Restoration Demonstration Project Meeting Pagosa Springs, Colorado. Online. Internet. 31 July 2002. Available: www.waterinfo.org/rio99b.htm.
Monitoring Restoration Effectiveness on the Lower Rio Blanco River
J.D. Kurz and David L. Rosgen Wildland Hydrology, Pagosa Springs, CO Lower Rio Blanco River near Pagosa Springs, Colorado Rio Blanco River
• Southwestern Colorado south of Pagosa Springs. • Drainage Area: 170mi2. • Tributary to the San Juan River. • U.S. Bureau of Reclamation has operated a major trans-basin diversion since 1971. • Approximately 70 % of annual water yield transmitted to Rio Grande Basin. Rio Blanco River Hydrograph
300 Pre-Diversion (1935-1971) USGS 09343000 (58 sq mi) 250 Post-Diversion (1971-2000) USGS 09343300 (69 sq mi) 200 2000 (Drought Year)
150
100
50 Mean Monthly Discharge (cfs)
0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Hydrology of the Rio Blanco River
• Diversion has decreased the magnitude and duration and altered the timing of the bankfull discharge. • Pre-diversion bankfull discharge: 726 cfs (1.5 year flow). • Post-diversion bankfull discharge: 463 cfs (new 1.5 year flow, a 36% reduction). • Flood water usually bypassed by diversion – August 24, 1992: 3120 cfs. Typical photograph showing the wide, shallow channel at low flow prior to restoration Consequences of Flow Depletion
Channel Stability
• Wide, shallow stream lacking the shear stress necessary route sediment supplied from unregulated tributaries.
• Aggradation and lateral migration.
• Increased flood stage. Consequences of Flow Depletion
Water Quality • Higher water temperatures and lower dissolved oxygen during summer low flow.
Aquatic Habitat • Poorly defined thalweg lacking the depth necessary to create good instream habitat for trout.
• Accumulation of fine sediment in spawning gravels during low flow. Restoration
• A collective effort by Rio Blanco Homeowners Association, Colorado Water Conservation Board, and several federal agencies.
• Section 319 of the Clean Water Act (EPA) provided partial funding (Hydrologic Modification is recognized as non-point source pollution in CO).
• A 1.1 mile demonstration reach was restored in the fall of 1999. Location of the Demonstration Reach
Project End Project Start Restoration Objectives
Construct a channel to match the altered flow regime in order to restore the physical and biological function of the river. • Improve sediment transport and flood capacity. • Reduce bank erosion. • Raise water table during low flow. • Decrease water temperature. • Increase dissolved oxygen. • Improve fish habitat. • Improve spawning gravel. • Improve aesthetics. Restoration Design
• Create a small, low width/depth ratio channel within the over-wide C3 and F3 stream types.
• Use material excavated from the low flow channel to construct floodplains and point bars.
• Use large boulders to construct J-Hook Vanes and Cross Vanes. Restoration Design – Typical Cross Section 93 Existing Channel 91 Bankfull Proposed Channel
89
87 Fill
Relative Elevation (ft) 85 F Cut
83 0 20406080100 Distance from Left Endpoint (ft) Channel Construction
Constructed Point Bar J-Hook Vanes Reduce bank erosion and maintain deep pools downstream of each structure.
Constructed Point Bar Cross Vanes Provide grade control, reduce bank erosion, and maintain deep pools downstream of each structure.
Constructed Floodplain Cross Vanes Provide bridge protection. Monitoring Restoration Effectiveness
• Permanent cross sections were installed following construction to monitor channel change. • A longitudinal profile was surveyed through the entire project length. • Resurveyed following the 2001 spring runoff , which provided excellent test of the restoration stability (1040 cfs). Longitudinal Profile through Cross Sections A, B, and C
6964
6962 XS-A XS-B 6960 XS-C
6958
6956 Thalweg 01 Elevation (ft) Elevation 6954 Water Surface 01 6952 Bankfull Bedrock 6950 J-Hook 6948 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 Distance from Start of Project Cross Section A
2001 Low Flow Dimensions Discharge Width Mean Depth Width/Depth Ratio Max Depth 18 cfs 22.0 ft 0.81 ft 27.1 1.6 ft
6962
6960
6958
6956 Elevation (ft) 6954
6952 0 102030405060 Distance from Left Endpoint (ft)
2001 2000 Water Surface Bankfull Cross Section B
2001 Low Flow Dimensions Discharge Width Mean Depth Width/Depth Ratio Max Depth 18 cfs 20.3 ft 0.81 ft 25.2 1.9 ft
6962
6960
6958
6956 Elevation (ft) 6954
6952 0 10203040506070 Distance from Left Endpoint (ft)
2001 2000 Water Surface Bankfull Cross Section C
2001 Low Flow Dimensions Discharge Width Mean Depth Width/Depth Ratio Max Depth 18 cfs 19.4 ft 0.83 ft 23.4 1.3 ft
6962
6960
6958
6956 Elevation (ft) 6954
6952 0 10203040506070 Distance from Left Endpoint (ft)
2001 2000 Water Surface Bankfull Longitudinal Profile through Cross Section 33+72
6956
6954 XS 33+72 6952
6950
6948
Elevation(ft) 6946 Thalweg 01 6944 Water Surface 01
6942 Bankfull J-Hook 6940 2700 2900 3100 3300 3500 Distance from Start of Project (ft) Cross Section 33+72
2001 Low Flow Dimensions Discharge Width Mean Depth Width/Depth Ratio Max Depth 18 cfs 18.6 ft 0.68 ft 27.5 1.3 ft
6954
6952
6950
6948 Elevation (ft) 6946
6944 0 102030405060 Distance from Left Endpoint (ft)
2001 2000 Water Surface Bankfull Longitudinal Profile through Cross Sections 40+00, 42+00, and 44+00
6948 XS 40+00 6946 XS 42+00 6944
6942 XS 44+00
(ft) 6940
6938
6936
Elevation Thalweg 01 6934 Water Surface 01 Bankfull 6932 J-Hook 6930 X-Vane 6928 3600 3800 4000 4200 4400 Distance from Start of Project (ft) Cross Section 40+00
2001 Low Flow Dimensions Discharge Width Mean Depth Width/Depth Ratio Max Depth 18 cfs 23.7 ft 1.68 ft 14.0 2.9 ft
6946
6944
6942
6940 Elevation (ft) 6938
6936 0 102030405060 Distance from Left Endpoint (ft)
2001 2000 Water Surface Bankfull Cross Section 42+00
2001 Low Flow Dimensions Discharge Width Mean Depth Width/Depth Ratio Max Depth 18 cfs 20.9 ft 0.72 ft 29.1 1.3 ft
6944
6942
6940
6938 Elevation (ft) 6936
6934 0 1020304050 Distance from Left Endpoint (ft)
2001 2000 Water Surface Bankfull Cross Section 44+00
2001 Low Flow Dimensions Discharge Width Mean Depth Width/Depth Ratio Max Depth 18 cfs 24.5 ft 1.09 ft 22.5 1.9 ft
6942
6940
6938
6936 Elevation (ft) 6934
6932 0 102030405060 Distance from Left Endpoint (ft)
2001 2000 Water Surface Bankfull Longitudinal Profile through Cross Sections 48+00 and 50+00
6940 XS 48+00 6938
6936 XS 50+00 6934 (ft) 6932
6930 Elevation 6928 Thalweg 01 Water Surface 01 6926 Bankfull 6924 J-Hook X-Vane 6922 4500 4700 4900 5100 5300 Distance from Start of Project (ft) Cross Section 48+00
2001 Low Flow Dimensions Discharge Width Mean Depth Width/Depth Ratio Max Depth 22 cfs 28.3 ft 1.4 ft 20.2 2.5 ft
6938
6936
6934
6932 Elevation (ft) 6930
6928 0 102030405060708090100 Distance from Left Endpoint (ft)
2001 2000 Water Surface Bankfull Cross Section 50+00
2001 Low Flow Dimensions Discharge Width Mean Depth Width/Depth Ratio Max Depth 22 cfs 27.7 ft 1.67 ft 16.6 2.6 ft
6936
6934
6932
6930
6928 Elevation (ft)
6926
6924 0 102030405060 Distance from Left Endpoint (ft)
2001 2000 Water Surface Bankfull Summary of Changes in Bankfull Area of 9 Cross Sections 200 Above the Line = Increase in 2001 160 ) 2 120
80 2001 Area (ft 2001 Area 40 Below the Line = Decrease in 2001 0 0 40 80 120 160 200
1999 or 2000 Area (ft2) Summary of Changes in Bankfull Width of 9 Cross Sections 70
60 Above the Line = Increase in 2001 50
40 30
2001 Width (ft) 2001 Width 20
10 Below the Line = Decrease in 2001 0 0 10203040506070 1999 or 2000 Width (ft) Summary of Changes in Bankfull Mean Depth of 9 Cross Sections 4 Above the Line = Increase in 2001 3
2
1 2001 Mean Depth (ft) Depth 2001 Mean Below the Line = Decrease in 2001 0 01234 1999 or 2000 Mean Depth (ft) Summary of Changes in Bankfull Width/Depth Ratio of 9 Cross Sections 35
30 Above the Line = Increase in 2001 25
20 15 10
2001 Width/Depth Ratio 5 Below the Line = Decrease in 2001 0 0 5 10 15 20 25 30 35 1999 or 2000 Width/Depth Ratio Summary of Changes in Bankfull Bankfull Max Depth of 9 Cross Sections 8 7 Above the Line = Increase in 2001 6 5 4 3 2 2001 (ft) Depth Max 1 Below the Line = Decrease in 2001 0 012345678 1999 or 2000 Max Depth (ft) Division of Wildlife Cross Sections (Surveyed Prior to Construction)
100 96 100 95 99 99 94 98 97 98 93 96 97 92 95 91 96 94 90 93 Relative Elevation (ft) Elevation Relative (ft) Elevation Relative Relative Elevation (ft) Elevation Relative 95 89 92 0 10203040506070 0 102030405060 0 102030405060 Distance from Left Endpoint (ft) Distance from Left Endpoint (ft) Distance from Left Endpoint (ft)
Channel Water Surface Bankfull Channel Water Surface Bankfull Channel Water Surface Bankfull
97 97 96 96 95 95 94 94 93 93 92
Relative Elevation (ft) Elevation Relative 92 (ft) Elevation Relative 91 0 10203040506070 0 10203040506070 Distance from Left Endpoint (ft) Distance from Left Endpoint (ft)
Channel Water Surface Bankfull Channel Water Surface Bankfull
Discharge Width* Mean Depth* Width/Depth Ratio* Max Depth* 33 cfs 37.1 ft 0.67 ft 57.9 1.0ft *Average values from 5 cross sections at 33 cfs Even with 33 percent less flow, the mean depths of the 2001 cross sections are greater than the average mean depth of 5 Division of Wildlife pre-restoration cross sections.
1.8 1.6 1.4 1.2 1 0.8 0.6
Mean Depth (ft) 0.4 0.2 0 XS-A XS-B XS-C 33+72 40+00 42+00 44+00 48+00 50+00
Mean depth 2001 cross section (18 – 22 cfs) Average mean depth of 5 DOW cross sections (33 cfs) Even with 33 percent less flow, the max depths of the 2001 cross sections are greater than the average max of 5 Division of Wildlife pre-restoration cross sections.
3 2.5
2 1.5 1 Max Depth (ft) 0.5 0 XS-A XS-B XS-C 33+72 40+00 42+00 44+00 48+00 50+00
Max depth 2001 cross section (18 – 22 cfs) Average max depth of 5 DOW cross sections (33 cfs) Longitudinal Profile 0+00 – 9+00 6982
6980
6978
6976
6974 Thalweg 99
Elevation (ft) Elevation 6972 Thalweg 01 Water Surface 01 6970 Bankfull Bedrock 6968 J-Hook X-Vane 6966 0 100 200 300 400 500 600 700 800 900 Distance from Start of Project (ft) Longitudinal Profile 9+00 – 18+00
6972
XS 14+60 6970
6968
(ft) 6966
6964 Thalweg 99 Elevation Thalweg 01 6962 Water Surface 01 Bankfull Bedrock 6960 J-Hook X-Vane 6958 900 1100 1300 1500 1700 Distance from Start of Project (ft) Longitudinal Profile 18+00 – 27+00
6964
6962 XS-A XS-B 6960 XS-C
6958
6956 Thalweg 99
Elevation (ft) 6954 Thalweg 01 Water Surface 01 6952 Bankfull 6950 Bedrock J-Hook 6948 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 Distance from Start of Project Longitudinal Profile 27+00 – 36+00
6956
6954
XS 33+72 6952
6950
6948
Elevation (ft) Elevation 6946 Thalweg 99 6944 Thalweg 01 Water Surface 01 6942 Bankfull J-Hook 6940 2700 2800 2900 3000 3100 3200 3300 3400 3500 3600 Distance from Start of Project (ft) Longitudinal Profile 36+00 – 45+00
6948 XS 40+00 6946 XS 42+00 6944 XS 44+00 6942
6940
6938
6936 Thalweg 99 Elevation (ft) 6934 Thalweg 01 Water Surface 01 6932 Bankfull 6930 J-Hook X-Vane 6928 3600 3800 4000 4200 4400 Distance from Start of Project (ft) Longitudinal Profile 45+00 – 54+00
6940 XS 48+00 6938
6936 XS 50+00 6934 (ft) 6932
6930 Elevation 6928 Thalweg 99 Thalweg 01 6926 Water Surface 01 Bankfull 6924 J-Hook X-Vane 6922 4500 4700 4900 5100 5300 Distance from Start of Project (ft) Longitudinal Profile Permanent Stationing Points (Every 200 feet)
60 Depth Increase Pool 40 Riffle Run 20
0
-20
-40 Depth Decrease
% Change from As-Built Depth (ft) Depth As-Built Change from % -60 0 1000 2000 3000 4000 5000 6000 Distance from Start of Project (ft) Comparing Velocity Distribution at 2 Cross Sections
Unrestored Cross Section on an Outside Bend
6921 6920
Elevation (ft) 10 15 20 25 30 35 40 45 50 Distance from Left Endpoint 5 4.5 Restored Cross Section with J-Hook 4 6956 3.5 3 6955 2.5 6954 2 Elevation (ft)
10 15 20 25 30 35 1.5 (ft/s) Velocity Distance from Left Endpoint 1 0.5 0 Below the Restored Reach: Poorly Formed Thalweg (0.5 feet deep at 18 cfs). 2.5-foot Pool Depth below a J-Hook Vane (18cfs). 4.5-foot Pool Depth below a Cross Vane (18 cfs).