The -12 Mile Slough - Revision history

Cellsworth at 21:16, 18 June 2026

2026-06-18T21:16:32Z

Cellsworth at 23:03, 23 October 2025

2025-10-23T23:03:56Z

Cellsworth at 19:29, 21 October 2025

2025-10-21T19:29:38Z

Cellsworth at 19:18, 21 October 2025

2025-10-21T19:18:27Z

Cellsworth at 22:04, 21 July 2025

2025-07-21T22:04:02Z

Cellsworth at 21:46, 11 June 2025

2025-06-11T21:46:57Z

Cellsworth at 21:19, 11 June 2025

2025-06-11T21:19:25Z

Cellsworth at 21:16, 11 June 2025

2025-06-11T21:16:48Z

Cellsworth at 16:13, 11 June 2025

2025-06-11T16:13:48Z

Cellsworth at 17:37, 4 February 2025

2025-02-04T17:37:43Z

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 |style="color:#000;"|
 *[https://www.usbr.gov/uc/progact/amp/amwg/2025-08-21-amwg-meeting/uc-gcdamp-amwg-sloughmodificationoverview-508-20250821.pdf Slough Modification Overview]
 *[https://www.usbr.gov/newsroom/news-release/5172?filterBy=region&region=Upper%20Colorado%20Basin Reclamation News Release: Channelization Project to Help Protect Native Fish in Grand Canyon Completed]

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 |style="color:#000;"|
 *[https://www.usbr.gov/newsroom/news-release/5172?filterBy=region&region=Upper%20Colorado%20Basin Reclamation News Release: Channelization Project to Help Protect Native Fish in Grand Canyon Completed]
 *[https://www.usbr.gov/uc/progact/amp/twg/2024-10-30-twg-meeting/20241030-GeomorphicBackgroundGlenCanyonSlough-508-AMWQD.pdf Geomorphic background for Glen Canyon Slough]

← Older revision		Revision as of 19:29, 21 October 2025
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	[[file:1889slough.png\|center\|450px\|]] <br>		[[file:1889slough.png\|center\|450px\|]] <br>
		+
		+	[[File:GSF Sloughs Rainstorm Sep-2-2016.jpg\|center\|700px\|thumbnail ]]
		+	The sloughs during a severe thunderstorm and flash flood event in 2016.

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-Modification of the -12 Miles Slough was completed the spring of 2025.
+==The -12 Mile Slough ==
-Two sloughs associated with a gravel bar that is 3.5 river miles downstream from Glen Canyon Dam or 12 river miles upstream from Lees Ferry, Arizona (AZ) (RM -12; Figure 1) are warmer than the Colorado River for much of the year creating habitat for warmwater non-native fishes. The lower (downstream) slough has a surface water connection with the Colorado River. However, the upper (upstream) slough is normally only connected with the Colorado River via groundwater through the sediments of the gravel bar (Figure 2). Darcy’s Law suggests this groundwater connection takes weeks to exchange water with the upper slough. At a river discharge of at least 20,000 cubic-feet-per-second (cfs; high end of daily fluctuating flow) water flows over the gravel bar and establishes a temporary surface water connection between the Colorado River and the upper slough. [https://www.usbr.gov/uc/progact/amp/amwg/2023-08-17-amwg-meeting/20230817-GlenCanyonSloughProposedModifications-508-UCRO.pdf]
+Two sloughs associated with a gravel bar that is 3.5 river miles downstream from Glen Canyon Dam or 12 river miles upstream from Lees Ferry, Arizona (AZ) (RM -12; Figure 1) are warmer than the Colorado River for much of the year creating habitat for warmwater non-native fishes. The lower (downstream) slough has a surface water connection with the Colorado River. However, the upper (upstream) slough is normally only connected with the Colorado River via groundwater through the sediments of the gravel bar (Figure 2). At a river discharge of at least 20,000 cubic-feet-per-second (cfs; high end of daily fluctuating flow) water flows over the gravel bar and establishes a temporary surface water connection between the Colorado River and the upper slough. [https://www.usbr.gov/uc/progact/amp/amwg/2023-08-17-amwg-meeting/20230817-GlenCanyonSloughProposedModifications-508-UCRO.pdf]
 |}<!--
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 |style="color:#000;"|
 *[https://www.usbr.gov/uc/progact/amp/twg/2024-10-30-twg-meeting/20241030-GeomorphicBackgroundGlenCanyonSlough-508-AMWQD.pdf Geomorphic background for Glen Canyon Slough]
 *[[Media:ZUNITHPO_LETTER_TO_AZSHPO_-12MILESLOUGHNPSACTION_13DECEMBER2024.pdf|Zuni THIO Letter to AZ SHPO regarding the -12 Mile Slough Modification]]
@@ Line 116: / Line 121: @@
 *November 2-6, 2015: rotenone treatment for green sunfish
 *September 17-19, 2022: rotenone treatment for smallmouth bass
-*August 25-28, 2023: herbicide treatment for smallmouth bass
+*August 25-28, 2023: herbicide treatment to remove aquatic vegetation used by smallmouth bass for cover
 |-

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 </table>
-[[file:Slough.jpg|thumb|center|450px|]] <br>
+[[file:Slough.jpg|center|450px|]] <br>
 <!--
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 Action is needed to disrupt or prevent the establishment of high-risk (predatory) warmwater nonnative fish in the proposed project area by limiting additional recruitment before the 2025 reproductive season and beyond. Warmwater nonnative fish threaten populations of the federally listed fish species humpback chub and razorback sucker and other native fish, amphibians, and invertebrates in Glen Canyon, Marble Canyon, and Grand Canyon. [https://parkplanning.nps.gov/document.cfm?parkID=62&projectID=123839&documentID=139804]
-[[file:SloughModification.png|thumb|center|600px|]] <br>
+[[file:SloughModification.png|center|600px|]] <br>
-[[file:SloughBeforeAfter.png|thumb|center|600px|]] <br>
+[[file:SloughBeforeAfter.png|center|600px|]] <br>
 |-
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 Following the closure of Glen Canyon Dam in 1963, Lake Powell began trapping sediment that resulted in erosion of the Colorado River channel through Glen Canyon creating a narrower and deeper channel (Pemberton, 1976, and Grams et al, 2007). Near RM -12, the channel thalweg has lowered 10 ft and the water surface elevation has lowered 5 ft subsequent to 1959 (Grams et al, 2007 see figure 9 of that report). Notations on a 1952 aerial photograph indicate that the left side of the river channel was shallow with a submerged “gravel and rock bar” at the upstream end and a sandbar at the downstream end (Figure 3A). In other words, the upper and lower slough as they exist today did not exist prior to the dam. After many years of streambed erosion, the channel became deeper along the right side of the canyon while the left side of the former riverbed became exposed as a high gravel bar. Depressions on the left side of the exposed gravel bar now function as the upper and lower sloughs (Figure 3B). [https://www.usbr.gov/uc/progact/amp/amwg/2023-08-17-amwg-meeting/20230817-GlenCanyonSloughProposedModifications-508-UCRO.pdf]
-[[file:Aerialslough.png|thumb|center|450px|]] <br>
+[[file:Aerialslough.png|center|450px|]] <br>
 The 1889 photo, captured 74 years before 1963 dam completion, shows a sandbar and a distant backwater that have persisted in various forms during the past 135 years or longer. The bar is composed of sand with no vegetation, suggesting a more dynamic river with high spring season flows and summer monsoon flash floods, which at times flowed over the bar. The 2011 photo, captured 61 years after dam completion, shows the slough site with cobble and vegetation growth, suggesting a less dynamic river with less frequent flows of water over the bar. During the 135-year period, the general shape of the bar and lower slough backwater remain similar, even though river flows at times covered the entire slough and bar with high water flows. [https://parkplanning.nps.gov/document.cfm?parkID=62&projectID=123839&documentID=139804]
-[[file:1889slough.png|thumb|center|450px|]] <br>
+[[file:1889slough.png|center|450px|]] <br>
-−
-−
-|-
-! <h2 style="margin:0; background:#cedff2; font-size:120%; font-weight:bold; border:1px solid #a3b0bf; text-align:left; color:#000; padding:0.2em 0.4em;">The Slough Spring</h2>
-|-
-|style="color:#000;"|
-−
-[[file:SloughSpring.png|thumb|center|450px|]] <br>
-−
-[[file:SloughSpring2.png|thumb|center|450px|]] <br>
 |}
@@ Line 140: / Line 132: @@
 Upper slough water temperatures are more susceptible to seasonal fluctuations than the river. Water temperatures in the upper slough typically become even colder than the river water during the winter because there is not normally a surface water connection between the river and upper slough during this time of the year. The reverse pattern happens during the summer when the slough water temperatures warm with the air temperature. Discharge and temperature data from 2015 show that water temperatures in the lower slough vary with season and with river discharge (Figure 9). Water temperatures in the slough were colder than the river water during that year in January, February, much of March, November, and December. However, water temperatures in the lower slough were significantly warmer than the river channel during May, June, July, August, and September. Similar trends were observed in other abbreviated data sets provided by the National Park Service for 2013 through 2017, and 2022. Additionally, discharge and temperature data from August 2022 show that water temperatures in the lower slough were significantly warmer during weekends when the discharges are relatively steady, indicating that daily discharge fluctuations during the week exchange water with the lower slough and limit warming (Figure 10). [https://www.usbr.gov/uc/progact/amp/amwg/2023-08-17-amwg-meeting/20230817-GlenCanyonSloughProposedModifications-508-UCRO.pdf]
-[[file:SloughTemperatures.png|thumb|center|450px|]] <br>
+[[file:SloughTemperatures.png|center|450px|]] <br>
-[[file:SloughTemperaturesBugFlows.png|thumb|center|450px|]] <br>
+[[file:SloughSpring2.png|center|450px|]] <br>
 |-

← Older revision		Revision as of 21:19, 11 June 2025
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	[[file:SloughSpring.png\|thumb\|center\|450px\|]] <br>		[[file:SloughSpring.png\|thumb\|center\|450px\|]] <br>
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		+	[[file:SloughSpring2.png\|thumb\|center\|450px\|]] <br>

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 |class="MainPageBG" style="width:55%; border:1px solid #cef2e0; background:#f5faff; vertical-align:top; color:#000;"|
 {|width="100%" cellpadding="2" cellspacing="5" style="vertical-align:top; background:#f5faff;"
-! <h2 style="margin:0; background:#cedff2; font-size:120%; font-weight:bold; border:1px solid #a3bfb1; text-align:left; color:#000; padding:0.2em 0.4em;">Is the slough a historical feature?</h2>
+! <h2 style="margin:0; background:#cedff2; font-size:120%; font-weight:bold; border:1px solid #a3bfb1; text-align:left; color:#000; padding:0.2em 0.4em;">Slough Modification</h2>
 |-
 |style="color:#000;"|
-Following the closure of Glen Canyon Dam in 1963, Lake Powell began trapping sediment that resulted in erosion of the Colorado River channel through Glen Canyon creating a narrower and deeper channel (Pemberton, 1976, and Grams et al, 2007). Near RM -12, the channel thalweg has lowered 10 ft and the water surface elevation has lowered 5 ft subsequent to 1959 (Grams et al, 2007 see figure 9 of that report). Notations on a 1952 aerial photograph indicate that the left side of the river channel was shallow with a submerged “gravel and rock bar” at the upstream end and a sandbar at the downstream end (Figure 3A). In other words, the upper and lower slough as they exist today did not exist prior to the dam. After many years of streambed erosion, the channel became deeper along the right side of the canyon while the left side of the former riverbed became exposed as a high gravel bar. Depressions on the left side of the exposed gravel bar now function as the upper and lower sloughs (Figure 3B). [https://www.usbr.gov/uc/progact/amp/amwg/2023-08-17-amwg-meeting/20230817-GlenCanyonSloughProposedModifications-508-UCRO.pdf]
+.2 PURPOSE AND NEED FOR PROPOSED ACTION
-[[file:Aerialslough.png|thumb|center|450px|]] <br>
+Purpose: The purpose of taking action is to compliment collaborative, interagency management strategies being implemented to prevent the successful reproduction of high-risk (predatory) warmwater nonnative fish, particularly smallmouth bass and green sunfish in the upper and lower sloughs at river mile -12, which has conditions favorable for nonnative fish spawning. The approach would seek to minimize negative effects on native species, restore natural functions or dynamics, and mitigate impacts, as necessary.
-The 1889 photo, captured 74 years before 1963 dam completion, shows a sandbar and a distant backwater that have persisted in various forms during the past 135 years or longer. The bar is composed of sand with no vegetation, suggesting a more dynamic river with high spring season flows and summer monsoon flash floods, which at times flowed over the bar. The 2011 photo, captured 61 years after dam completion, shows the slough site with cobble and vegetation growth, suggesting a less dynamic river with less frequent flows of water over the bar. During the 135-year period, the general shape of the bar and lower slough backwater remain similar, even though river flows at times covered the entire slough and bar with high water flows. [https://parkplanning.nps.gov/document.cfm?parkID=62&projectID=123839&documentID=139804]
+Need: The need is driven by increased levels of entrainment of high-risk nonnative warmwater species surviving passage through the Glen Canyon Dam due to lower Lake Powell elevations, the increased dam outflow temperatures in 2022 and 2023, the expectation that these conditions will occur in the future, and the resulting reproduction of predatory smallmouth bass and green sunfish, including in the river mile -12 upper and lower slough. Reducing nonnative fish reproduction at this site would help reduce the likelihood of their downstream expansion and establishment in Glen, Marble, and Grand Canyons.
-[[file:1889slough.png|thumb|center|450px|]] <br>
+Action is needed to disrupt or prevent the establishment of high-risk (predatory) warmwater nonnative fish in the proposed project area by limiting additional recruitment before the 2025 reproductive season and beyond. Warmwater nonnative fish threaten populations of the federally listed fish species humpback chub and razorback sucker and other native fish, amphibians, and invertebrates in Glen Canyon, Marble Canyon, and Grand Canyon. [https://parkplanning.nps.gov/document.cfm?parkID=62&projectID=123839&documentID=139804]
 |-
-! <h2 style="margin:0; background:#cedff2; font-size:120%; font-weight:bold; border:1px solid #a3b0bf; text-align:left; color:#000; padding:0.2em 0.4em;">Slough Modification</h2>
+! <h2 style="margin:0; background:#cedff2; font-size:120%; font-weight:bold; border:1px solid #a3b0bf; text-align:left; color:#000; padding:0.2em 0.4em;">Is the slough a historical feature?</h2>
 |-
 |style="color:#000;"|
-.2 PURPOSE AND NEED FOR PROPOSED ACTION
+Following the closure of Glen Canyon Dam in 1963, Lake Powell began trapping sediment that resulted in erosion of the Colorado River channel through Glen Canyon creating a narrower and deeper channel (Pemberton, 1976, and Grams et al, 2007). Near RM -12, the channel thalweg has lowered 10 ft and the water surface elevation has lowered 5 ft subsequent to 1959 (Grams et al, 2007 see figure 9 of that report). Notations on a 1952 aerial photograph indicate that the left side of the river channel was shallow with a submerged “gravel and rock bar” at the upstream end and a sandbar at the downstream end (Figure 3A). In other words, the upper and lower slough as they exist today did not exist prior to the dam. After many years of streambed erosion, the channel became deeper along the right side of the canyon while the left side of the former riverbed became exposed as a high gravel bar. Depressions on the left side of the exposed gravel bar now function as the upper and lower sloughs (Figure 3B). [https://www.usbr.gov/uc/progact/amp/amwg/2023-08-17-amwg-meeting/20230817-GlenCanyonSloughProposedModifications-508-UCRO.pdf]
-Purpose: The purpose of taking action is to compliment collaborative, interagency management strategies being implemented to prevent the successful reproduction of high-risk (predatory) warmwater nonnative fish, particularly smallmouth bass and green sunfish in the upper and lower sloughs at river mile -12, which has conditions favorable for nonnative fish spawning. The approach would seek to minimize negative effects on native species, restore natural functions or dynamics, and mitigate impacts, as necessary.
+[[file:Aerialslough.png|thumb|center|450px|]] <br>
-Need: The need is driven by increased levels of entrainment of high-risk nonnative warmwater species surviving passage through the Glen Canyon Dam due to lower Lake Powell elevations, the increased dam outflow temperatures in 2022 and 2023, the expectation that these conditions will occur in the future, and the resulting reproduction of predatory smallmouth bass and green sunfish, including in the river mile -12 upper and lower slough. Reducing nonnative fish reproduction at this site would help reduce the likelihood of their downstream expansion and establishment in Glen, Marble, and Grand Canyons.
+The 1889 photo, captured 74 years before 1963 dam completion, shows a sandbar and a distant backwater that have persisted in various forms during the past 135 years or longer. The bar is composed of sand with no vegetation, suggesting a more dynamic river with high spring season flows and summer monsoon flash floods, which at times flowed over the bar. The 2011 photo, captured 61 years after dam completion, shows the slough site with cobble and vegetation growth, suggesting a less dynamic river with less frequent flows of water over the bar. During the 135-year period, the general shape of the bar and lower slough backwater remain similar, even though river flows at times covered the entire slough and bar with high water flows. [https://parkplanning.nps.gov/document.cfm?parkID=62&projectID=123839&documentID=139804]
-Action is needed to disrupt or prevent the establishment of high-risk (predatory) warmwater nonnative fish in the proposed project area by limiting additional recruitment before the 2025 reproductive season and beyond. Warmwater nonnative fish threaten populations of the federally listed fish species humpback chub and razorback sucker and other native fish, amphibians, and invertebrates in Glen Canyon, Marble Canyon, and Grand Canyon. [https://parkplanning.nps.gov/document.cfm?parkID=62&projectID=123839&documentID=139804]
+[[file:1889slough.png|thumb|center|450px|]] <br>
 |-

@@ Line 94: / Line 94: @@
 *[https://parkplanning.nps.gov/projectHome.cfm?parkID=62&projectID=123839 NPS EA Page: Warm Water Invasive Fish Management at the -12 Mile Slough]
 *[https://parkplanning.nps.gov/document.cfm?parkID=62&projectID=123839&documentID=139804 2024 NPS Slough EA: Warmwater Nonnative Fish Management Plan / Environmental Assessment ]
 *[https://www.usbr.gov/uc/progact/amp/amwg/2023-08-17-amwg-meeting/20230817-GlenCanyonSloughProposedModifications-508-UCRO.pdf 2023 Reclamation Proposed Modifications Report for Glen Canyon Sloughs]