Measurements were made before and 

after the experimental flood, March 

26ÐApril 3, 1996. DISCHARGE, IN CUBIC FEET PER SECOND

22 23 24 25 26 27 28 29 30 31 1 2 3

MARCH APRIL

4 5

50,000

0

40,000

30,000

20,000

10,000

Precontrolled flood data

Postcontrolled flood data

Photosynthesis and respiration by 

the plant community attached to the 

streambed, which includes rooted 

plants, algae, and bacteria, influence 

the daily variation in concentrations 

of oxygen and carbon dioxide in the 

water of the Colorado River just 

downstream from Glen Canyon Dam.

The magnitude of daily variation 

in oxygen and carbon dioxide 

concentrations depends on the 

density and mass of the plants (the 

plant biomass) if other conditions 

that control the concentrations are 

the same.

The controlled flood of March 

1996 scoured some of the plant 

biomass from the river channel 

below the dam, and thus reduced the 

amount of daily variation in oxygen 

and carbon dioxide concentrations 

and pH.

The fact that the daily variation in 

oxygen concentration and pH was 

measurable immediately after the 

flood indicates that the flood left a 

significant amount of the plant 

biomass; therefore, the community 

recovered rapidly.

 The Colorado River just downstream 

from the Glen Canyon Dam is one of the 

most biologically productive reaches of 

the river. Because water released from 

the dam comes from deep in Lake 

Powell, it is clear and carries enough 

nutrients to support substantial growth of 

algae and higher aquatic plants. This 

basic primary production in water that is 

cold and clear year-round provides 

conditions that support a world-famous 

trout fishery.

 The biological processes of photosynthesis and respiration influence daily 

chemical change in the Colorado River. 

During daylight hours, photosynthesis 

by aquatic plants adds oxygen to the 

water and removes carbon dioxide. 

At night, respiration does the 

reverse—consumes oxygen and produces 

carbon dioxide. The daily range

of oxygen and carbon dioxide 

concentrations that results from these 

processes depends on the amount of 

photosynthesis and respiration that takes 

place in the river. Other conditions that 

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 060-99

October 1999

pH OR OXYGEN CONCENTRATION

Figure 1. The daily change in oxygen concentration and pH depends on the 

biomassÑthe density and mass of plants in the reach.

SUNRISE NOON SUNSET MIDNIGHT SUNRISE

LOW

BIOMASS

Effects of Flooding 

on Plant Production

Downstream from 

Glen Canyon Dam 

Grand Canyon Monitoring 

and Research Program

HIGH

BIOMASS Prepared in cooperation with the 

Bureau of Reclamation
— G. Richard Marzolf

Additional Information

Marzolf, E.R., Mulholland, P.J., and Steinman, A.D., 

1995, Improvements to the diurnal upstreamdownstream dissolved oxygen change technique 

for determining whole-stream metabolism in 

small streams: Canadian Journal of Fisheries and 

Aquatic Sciences, v. 51, no. 7, p. 1591–1599.

Marzolf, G.R., Bowser, C.W., Hart, R.J., Stevens, 

D.W., and Vernieu, W.S., 1999, Photosynthetic 

and respiratory processes — An open-stream 

approach, in Webb, R.H., Schmidt, J.C., and 

Valdez, R.A., eds., The controlled flood 

 in Grand Canyon: American Geophysical

 Union, Monograph No. 110, p. 205–215.

G. Richard Marzolf

U.S. Geological Survey

National Center, MS432

Reston, VA 20192

(703) 648-5828

0

0 10 KILOMETERS

10 MILES

GLEN

CANYON

DAM

LEES

FERRY

River

36¡52'30" 

37¡

Paria

OXYGEN CONCENTRATION,

IN MILLIGRAMS PER LITER

pH

10

9

8

8.25

8.05

7.95

8.15

8.35

7

11

After flood

Before flood

The daily change in pH and oxygen 

concentration after the controlled flood 

was smaller than before the flood but 

was still measurable. 

10 AM 4 PM 4 AM 10 PM 10 AM

89

influence photosynthesis are length 

of day, amount of sunlight, condition 

of the plants, depth and clarity of 

the water, and nutrient availability.

The addition or removal of carbon 

dioxide causes a measurable change 

in the pH (a measure of the acidity) 

of the water. The pH increases when 

carbon dioxide is removed from the 

water during photosynthesis and 

decreases when carbon dioxide is 

added during respiration. Thus, a 

second way to assess these processes 

is to measure the pH, which is useful, 

because pH and oxygen concentrations 

can be measured by automated sensors.

The daily change in concentrations 

is larger at low river discharge simply 

because the same amount of plant 

material is causing changes in a 

smaller volume of water. At high 

discharges, the chemical change per 

unit volume may be so small as to 

be unmeasurable.

After flood

Before flood

Lake

Powell

Lake

Powell

30' 111¡ 15'

GLEN CANYON

 The design of the flood experiment 

included low-flow periods for 3 days 

before the high release and for 3 

days after the high release. This experiment was done primarily to 

document changes in sediment 

distribution in the Grand Canyon; 

it was, however, an extraordinary 

opportunity to assess the basic biological processes in this reach of the river 

under the same flow conditions before 

and after the flood. The working 

hypothesis was that if the flood scoured 

plants from the river, then the daily 

range of pH and oxygen concentration 

would be decreased (fig.1). If there were 

no scouring, then the daily range would 

 Measurements were made for 24-hour 

periods during the low flows and before 

and after the high flows. Oxygen 

measurements were made with three 

different methods at 0.5-hour intervals 

at Lees Ferry and at 3-hour intervals in 

the lower half of Glen Canyon between 

Glen Canyon Dam and Lees Ferry. 

Likewise, pH measurements were made 

with two methods at the same times and 

places. Measurements of the dissolvedcarbon chemistry also were made at 4-

hour intervals to be redundant and to 

ensure that no measurements were 

missed and that data could be crosschecked. Light intensity was measured 

during each 24-hour period in case 

there was a difference in cloud cover. A 

difference did occur with less light 

during the "before" measurements, but 

the difference was small. 

Some Results

 Measurements of pH and oxygen 

concentration at Lees Ferry before and 

after the flood show that the daily range 

was about 50 to 80 percent smaller 

immediately after the flood, which 

indicates that the flood scoured plants 

from the channel. This result was not 

unexpected. Of greater interest, 

however, is the fact that the daily 

increase and decrease in pH and oxygen 

concentration was still easily 

measurable after the flood, showing that 

substantial plant biomass was still in 

place. Recovery was not only possible, 

but it happened quickly. 

UTAH

ARIZONA

111¡45'

The Flood Experiment of 1996

Measurements of the Study

Measurements of pH and oxygen concentrations were made at Lees Ferry and 

in the lower half of Glen Canyon downstream from Glen Canyon Dam.

remain the same. In either case, the 

information would be useful to fishery 

managers.

For more information contact:

Odum, H.T., 1956, Primary production in flowing 

 waters: Limnology and Oceanography, v. 1, 

 p. 102–117.