Introduction 

Water stored behind, under, or 

within glaciers can be released rapidly 

and unexpectedly. These outburst 

floods can pose a serious threat to life 

and property. Post and Mayo (1971) 

identified glacier-dammed lakes in 

southeastern Alaska and in adjacent 

areas of Canada, and characterized the 

potential hazards associated with outburst floods. Tulsequah Lake in Canada (fig. 1) was identified as one of 

these glacier-dammed lakes with a 

long history of annual outburst floods. 

Tulsequah Lake is at about 1,140 

feet above sea level on the north side 

of Devils Paw Mountain. The lake is 

impounded by a distributary branch of 

Tulsequah Glacier and is about 20 

miles upstream from the mouth of the 

Tulsequah River, a major tributary to 

the Taku River. The U.S. Geological 

Survey (USGS) has operated a 

streamflow-gaging station on the 

Taku River (station No. 15041200) 

since 1987. 

Outburst floods from Tulsequah 

Lake have occurred frequently and 

have been documented by Marcus 

(1960), Miller (1963), Post and Mayo 

(1971), and the USGS (1988-99) 

(table 1). Tulsequah Lake is filled during the summer months by rainfall and 

by glacial melt. As Tulsequah Lake 

fills, the lake surface rises until the 

hydrostatic pressure at the base of the 

ice dam causes the ice to float. As this 

occurs, water begins flowing near the 

base of the glacier. This flowing water 

thermally erodes the dam, progressively enlarging the opening. At 

Tulsequah Lake, no surface drainage 

is evident, and all discharge appears to 

be through a subglacial tunnel. Data 

from documented outburst floods 

indicate that the flow of the Taku 

River generally doubles or triples during a 2- to 3-day period.

U.S. Geological Survey USGS Fact Sheet 131-99

U.S. Department of the Interior July 1999

Measurements of Taku River, Alaska 

During Tulsequah Lake 

Outburst Floods

Table 1. Dates of Tulsequah Lake outbursts

Year Date of outburst Year Date of outburst Year Date of outburst

1998 July 31–August 3 1987 August 25–28 1948 July 23–27

1997 July 25–28 1958 July 7–10 1947 August 5–9

1996 September 17–20 1957 August 13–16 1946 August 4–8

1995 July 24–27 1956 August 29–September 1 1945 August 8–11

1994 July 28–August 1 1955 September 4–7 1944 August 15–19

1993 July 26–29 1954 September 11–14 1943 July

1992 August 18–21 1953 July 6–10 1942 July

1991 August 30–September 2 1952 August 6–9 1932 August 15–20

1990 July 18–21 1951 July 26–29 1926 January

1989 August 14–18 1950 July 27–30 1910 Summer

1988 July 31–August 2 1949 August 7–10

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

Study

Area CANADA

ALASKA

JUNEAU

DOUGLAS ISLAND

STEPHENS PASSAGE

STEPHENS PASSAGE

Gastineau Channel

TAKU

Taku River

Tulsequah River

Tulsequah Glacier

Tulsequah Lake

Devils Paw

X

Canyon

Island

▲

 USGS

Stream-gaging

station 15041200 Taku

Lodge

Tulsequah 

Chief Mine

Figure 1. Location of Tulsequah Lake and Glacier and the USGS gaging station

on the Taku River, southeast Alaska.

ALASKA

 INLET

Taku

River

Proposed

road to

Atlin, B.C.

58o

15'

58o

30'

58o

45'

59o

135o 134o

30' 134o 133o

30'
1997 Tulsequah Lake Outburst 

Flood

An outburst flood from Tulsequah 

Lake was documented on the Taku River 

during July 25–28, 1997. USGS personnel 

from Juneau measured the discharge, 

stage, and temperature of the water surface 

at the gaging station on the Taku River. 

For the first time since the gaging station 

was established in 1987, measurements 

were made at the actual time of an outburst 

flood. Four measurements were made 

while flood stage and discharge were 

increasing, one measurement was made 

during the peak discharge, and three measurements were made while stage and discharge were decreasing (fig. 2). The prebreakout discharge was about 35,000 

cubic feet per second on July 24, and the 

peak discharge was about 70,000 cubic 

feet per second. 

An Acoustical Doppler Current Profiler (ADCP) was used to measure the discharge during the 1997 outburst flood. The 

ADCP is an improved method of making 

high-quality discharge measurements 

(Morlock, 1996). Discharge measurements 

made with the ADCP take less time, are 

safer, and have smaller errors than those 

made by conventional methods. The 

decreased time required for measuring the 

discharge is important for making highquality measurements during a rapidly 

changing stage. Furthermore, the positiontracking capability of the ADCP permits 

the measurements to be made without need 

for a cable across the flood path and allows 

the boat to be maneuvered to avoid debris 

carried by the river during the flood. 

The ADCP also recorded temperatures of the water surface during the eight 

discharge measurements. Water temperature of the Taku River decreased as Tulsequah Lake drained (table 2) and then 

increased slightly—but not uniformly 

across the river —during the peak of the 

outburst flood. Temperature continued to 

increase and become more uniform as the 

water discharge decreased. This temperature distribution probably indicates that the 

Taku River waters were not well mixed and 

that cooler outburst waters were concentrated on the north side of the river. The 

length of river that is incompletely mixed 

appears to extend as far as the gaging station, 8.5 miles downstream from the confluence of the Tulsequah and Taku Rivers. 

A similar temperature distribution was 

observed in the 1998 flood, during which 

the peak discharge was 69,000 cubic feet 

per second.

Flood Hazards

The annual glacier outburst of Tulsequah Lake is a potentially dangerous and 

destructive flood that may affect inhabitants of the Taku and Tulsequah River Valleys. As this area becomes developed, the 

frequency and magnitude of outburst 

floods are of great concern. 

The Tulsequah Chief Mine (fig. 1) is 

approximately 18.5 miles upstream from 

the Alaska–Canada border in the Tulsequah River Valley. Recently, the Canadian 

government has approved permits to reactivate the mine and build a 100-mile-long 

road to access the mine from Atlin, British 

Columbia. Outburst floods could damage 

the mine and road. About 40 cabins have 

been built along both banks of the river 

near the USGS stream-gaging station. In 

addition, increased tourism routinely 

brings visitors to the Taku River and Taku 

Lodge by boat and floatplane during the 

summer months. The floodwaters and 

debris released during the outburst floods 

can threaten the safety of tourists and have 

negative consequences on the fishing 

industry in Taku Inlet. Deposits of debris 

and sediments associated with the outburst 

may dramatically change the river channel 

during and after the Tulsequah Lake outburst floods. These channel changes create 

potentially hazardous conditions for an 

unknowing boat operator or floatplane 

pilot.

Understanding how rivers can rise 

unexpectedly and rapidly during an outburst flood improves the ability to identify 

possible downstream hazards and 

increases awareness. Understanding how 

discharge and water temperature vary during an outburst flood allows a better comprehension of the physical processes that 

released the water, which improves definition of downstream hazards and water 

quality.

References Cited

Marcus, M.G., 1960, Periodic drainage of glacier-dammed Tulsequah Lake, British

Columbia: Geographical Review, v. 50, p.

89-106.

Miller, M.M., 1963, Taku Glacier evaluation

study: State of Alaska Department of

Highways, 200 p.

Morlock, S.E., 1996, Evaluation of acoustic

Doppler current profiler measurements of

river discharge: U.S. Geological Survey

Water-Resources Investigations Report

95-4218, 37 p.

Post, Austin, and Mayo, L.R., 1971, Glacierdammed lakes and outburst floods in

Alaska: U.S. Geological Survey Hydrologic Atlas HA-455, 10 p., 3 pl.

U.S. Geological Survey, 1988-99, Water

resources data for Alaska—water years

1987-98: U.S. Geological Survey WaterData Reports AK-87-1 to AK-98-1 (published annually).

Table 2. Water temperature of the Taku 

River, July 25-28, 1997

Date

July 1997 Time 

Water temperature

(degrees Celsius)

South 

bank

North 

bank

25 9:29 p.m. 8 8

26 11:02 a.m. 5.5 4.5

8:03 p.m. 7 4.5

27 6:57 a.m. 6 2.5

2:36 p.m. 8.5 4.5

7:11 p.m. 9.5 6

10:27 p.m. 8 7.5

28 1:37 p.m. 8.5 8.5

5 7 9 11 13 15 17 19 21 23 25 27 29 31 2 4

July August

20,000

75,000

25,000

30,000

35,000

40,000

45,000

50,000

55,000

60,000

65,000

70,000

DISCHARGE, IN CUBIC FEET PER SECOND

 Discharge measurement

Figure 2. Discharge in the Taku River from July 5 to 

August 5, 1997. Tulsequah Lake outburst flood was 

on July 25-27.

—by Randy H. Host and 

Joseph M. Dorava

For more information, please

contact:

U.S. Geological Survey

4230 University Dr.

Anchorage, AK

99508-4664

907-786-7100

http://ak.water.usgs.gov