Distribution and Impacts of 

Benthic and Hyporheic Anoxia 

on the Colorado River 

Ecosystem Downstream from 

Glen Canyon Dam, Arizona

Courtney McDaniel

Larry Stevens

Joseph Holway

Craig Ellsworth
Springs Stewardship Institute

• Museum of Northern Arizona: 50 years of 

preserving the heritage of the Colorado 

Plateau

• Springs Stewardship Institute: focus on spring 

ecosystem ecology & conservation

– Director: Dr. Larry Stevens
Glen Canyon Dam & Tailwaters

• Hypolimnetic flow – cold, low oxygen

• Very little sediment 

• Highly controlled releases – predictable river stages

Lees Ferry
Benthic & Hyporheic Anoxia (BHA)

• Hyporheic zone = river 

sediments where surface and 

groundwater are exchanged

• Benthic zone = shallowest 

portion of hyporheic zone

• Anoxia = lacking oxygen

• Characteristics:

– Stinky!

– Fine sand/organic particles

– Just below the surface 

sediments

– AKA “maenke”
Benthic & Hyporheic Anoxia

• Common in many 

stagnant-water habitats

– Swamps

– Floodplains

– Bogs 

• Due to decomposition 

– Bacteria decompose leaf 

litter  uses up oxygen

– With little water flow, no 

new oxygen introduced 

ANOXIA

• BUT, not documented in 

flowing rivers or below 

dams... until now
Study Objectives

1. Determine distribution of BHA

– Lower Lake Powell

– GC Dam to Lees Ferry

– Below Lees Ferry

2. Explore what may be causing BHA to form

3. Determine if BHA has negative effects on 

aquatic insects
1. Distribution of BHA

LP1

LP2

LP3

CR-15

CR-12

CR-6.5 CR-9

CR-3

CRBLF1

CRBLF3

CRBLF2

CRLF

GC Dam

Lees Ferry
1. Distribution of BHA

Site transects:

• Transects extended from highest 

elevation point on bank to 1.5 m below 

low water stage

• Elevation profiles

• Soil excavated at each abrupt change in 

elevation or substrate

• Presence/depth of BHA measured
1. Distribution of BHA

• Most prevalent: 

– between GC dam and Lees 

Ferry

– in slow-moving waters

– in the presence of dense 

submerged vegetation
1. Distribution of BHA

• Most prevalent: 

– between GC dam and Lees 

Ferry

– in slow-moving waters

– in the presence of dense 

submerged vegetation
1. Distribution of BHA

Below Paria River mouth

Lake Powell
1. Distribution of BHA

• Seasonality

– Greatest extent in 

early fall

– Decrease in colder 

months

– Appears to track 

water stage along 

shoreline
2. BHA Development

• Field experiment

– 20 L chambers with nonBHA river sediment + river 

water

– 6 floating, 6 submerged

– In each treatment: 3 with 

Chara algae added, 3 

without Chara

– 38 days

– Measured depth of BHA, 

water chemistry (temp, DO, 

pH)

Floating Submerged
2. BHA Development

• Results – field experiment

– BHA only in buckets with 

vegetation

– Significant BHA development in 38 

days

– Developed in both submerged and 

floating buckets

– Significantly lower DO in 

treatments with Chara (p<0.001)

• Without Chara: mean = 7.36 mg/L

• With Chara: mean = 5.71 mg/L
2. BHA Development

• Lab experiment

– How do temperature, 

aeration, & vegetation 

interact to affect BHA 

development?

– Treatments:

• 3 temperatures (5°C, 12°C, 

20°C)

– With Chara

– With Chara & aeration

– With aeration

– Neither Chara nor aeration

– 10 days

– Measured depth of BHA, 

water chemistry (DO, pH)
2. BHA Development

• Results – lab experiment

– All +Chara treatments developed 

BHA

– BHA development increased 

with each increase in 

temperature

– Non-aerated treatments 

developed more BHA than 

aerated treatments

– Even at elevated temp and with 

Chara present, aeration led to 

decreased BHA development
2. BHA Development

• Results – lab experiment

– Dissolved oxygen results similar to field experiment

– Presence of Chara led to lower DO

– Aeration + low temperature lessened effect of Chara presence

0

1

2

3

4

5

6

7

8

9

Chara + Chara - Chara + Chara -

Aerated Non-aerated

Mean Dissolved Oxygen (mg/L)

5°C 12°C 20°C
3. Effects on aquatic insects

Dragonflies

Damselflies

Mayflies
3. Effects on aquatic insects

High mortality in mayflies

y = -47.609x + 111.19

R2 = 0.9097

y = -23.644x + 115.04

R2 = 0.7369

0

10

20

30

40

50

60

70

80

90

100

0 0.5 1 1.5 2 2.5

% Survival

Log Time (hrs)

HA treatment

Control

n = 31 per treatment

www.first-nature.com
Conclusions

• BHA most prevalent in dam tailwaters

• BHA development limited by:

– Aeration

– Turbidity

– Lower temperature

– Decreased aquatic vegetation

• Some aquatic invertebrates may be impaired 

by BHA
Potential Solutions

• Increase oxygenation of the tailwaters  aeration of 

benthic sediment

– Fairly unrealistic

• Increase turbidity  less light for vegetation to grow  less 

decomposition

– BUT negative effects on trout fishery

– Expensive

• Decrease water temperature  less microbial activity

– BUT may harm native fish survivorship

• Increase flow variability  increase scour  expose 

sediments to air  decrease growth of aquatic vegetation 

(via exposure & turbidity)
Downstream

• Currently, mostly limited to Glen Canyon reach

• Potential in Grand Canyon

– Climate change: drier climate in SW U.S.  less 

tributary input  less turbidity  more vegetation 

more BHA
Thank you!

Thanks to:

Glen Canyon National Recreation Area

Glen Canyon Adaptive Management Program