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II. THE BEAR CREEK DRAINAGE BASIN

Drainage Basin Description

Bear Creek, which rises in the mountains southwest of Denver, is a left bank tributary of the South Platte River. The total drainage area at the mouth is 261 square miles of which 164 square miles are upstream of Morrison. The basin, shown in Figure II-1, includes parts of Jefferson, Clear Creek and Park Counties, and ranges in elevation from 5,780 feet at Morrison to 14, 264 feet at Mt. Evans. Idledale, Kittredge and Evergreen are towns located in Jefferson County along Bear Creek upstream of Morrison. Major tributaries entering Bear Creek below Evergreen Lake include: Cub Creek, Troublesome Creek, Swede Gulch, Cold Spring Gulch, Sawmill Gulch at Idledale and Mount Vernon Creek at Morrison. Bear Creek flows into Bear Creek Lake just east (downstream) of the Dakota Hogback geologic formation at Morrison. This facility is a major flood control reservoir constructed and operated by the U. S. Army Corps of Engineers. East of the Hogback, Rooney Gulch enters Bear Creek Lake from the north and Turkey Creek enters the lake from the south. The City of Lakewood Parks Department is responsible for public safety in the park area surrounding Bear Creek Lake. Evergreen Dam is a 380’ long, 34’ high structure located on the main stem of Bear Creek above Cub Creek at Evergreen, forming a 40-acre lake known as Evergreen Lake. This reservoir is not a flood control facility but it does impound 670 acre-feet of water.

Flood History

Information on historical flooding in the Bear Creek basin has been obtained from the following sources:

1. Flood Plain Information, Metropolitan Region, Denver, Colorado, Volume III Summary Report Bear and Clear Creeks, South Platte River Basin, and Volume III Technical Appendix, Bear and Clear Creeks, South Platte River Basin, U.S. Army Corps of Engineers (USACE), Omaha, Nebraska, January 1966.

2. Floodplain Information, Bear Creek and Mt. Vernon Creek, Morrison, Colorado, USACE, October 1971.

3. Special Flood Hazard Information Report, Bear Creek, Denver Metropolitan Area, USACE, December 1972.

Figure II-1
Bear Creek Drainage Basin

Additional historic flood information was obtained from the Jefferson County Department of Emergency Management and the Canyon Courier newspaper in Evergreen, Colorado.

Since the 1860’s there have been 24 known instances of floods in the Bear Creek basin that collectively have caused 45 deaths and considerable property damage. These floods occurred in 1866, two in 1878, 1885, 1886, 1893, 1894, 1896, 1903, 1907, 1908, two in 1921, 1923, 1925, 1933, 1934, 1938, 1946, two in 1957, 1965, 1969 and 1973. Most of the floods were caused by runoff from intense rainstorms during the summer months. However, early season floods have resulting from rainfall runoff coinciding with snowmelt flows. The Bear Creek at Morrison stream gage, located upstream of Mount Vernon Creek, measured its record flow of 8,600 cfs (estimated) on July 24, 1896. The peak discharge for Bear Creek during the 1938 flood was estimated to be 9,230 cfs at Morrison with most of the runoff coming from Mount Vernon Creek . There are no gage records for Mount Vernon Creek. The total drainage area at the Bear Creek at Morrison gage is 164 square miles. Mount Vernon Creek enters Bear Creek downstream of this gage and has a drainage area of only 9.4 square miles. The headwaters of Mount Vernon Creek are at Genesee where I-70 begins its climb into the mountains along Mount Vernon Canyon. The south side of Lookout Mountain also drains into Mount Vernon Creek. At the Dakota Hogback the creek turns south, passing through Red Rocks Park and continuing to its mouth at Morrison, where a very narrow, confined stream channel exists.

The stream gage at Sheridan, located at Lowell Blvd., has measured Bear Creek flows continuously since 1927. The Morrison gage has partial records dating back to 1888 and continuous records since 1922. Table II-1 lists the largest floods recorded at these gaging stations since 1927. Comparison of the gage records reveals a large variance in peak discharges for each flood event. In 1933, 1934 and 1938, the storms were concentrated in the foothills and mountains of Bear Creek, and the resulting flood peaks attenuated between Morrison and Sheridan. For the other flood dates shown in Table II-1, the majority of runoff occurred from watershed areas downstream of Morrison or from Turkey Creek. Thus the peak flows for these floods were greater at Sheridan than Morrison. Turkey Creek was the known principle contributor for the 1957, 1965, 1969 and 1973 events. The flood of 1973 was the last significant event in the Bear Creek basin.

TABLE II-1
BEAR CREEK PEAK FLOOD DISCHARGES
IN CFS

DATE

MORRISON

SHERIDAN

July 7, 1933

8,110

3,000

August 9, 1934

4,620

1,300

September 2, 1938

6,200

2,810

August 21, 1957

1,640

2,560

July 25, 1965

1,030

2,900

May 7, 1969

2,340

8,150

May 6, 1973

1,480

3,750

The 1971 USACE report characterizes Bear Creek floods as rapid concentrations of runoff, sharp peak discharges and rapid flood recession. The peaking time for floods on Bear Creek at Morrison is about 3 to 5 hours after the causing rainfall, while floods on Mount Vernon Creek can peak between 1 and 3 hours. Several local residents stated that during the Mount Vernon Creek flood of 2-3 September 1938, it was about 1.5 to 2 hours from the time the flood reached Morrison until the water had receded enough to permit travel on Main Street. A large flood on Bear Creek at Morrison due to intense rainfall could last up to 12 hours and the duration of flooding could be extended even more in the case of coincident high snowmelt runoff.

Some major past floods on Bear Creek and Mount Vernon Creek are described to illustrate the dangerous flood potential at Morrison. One of the earliest reports concerns a storm on 21-23 May, 1876, reported by the Denver Tribune on June 5 of that year; "... informs us that he has never seen such destruction in the region as resulted from the late storm. He spent some days in the valleys of Soda and Bear Creeks and their tributaries and found new gullies worn to the depth of 20 feet in the action of the raging torrents." Another report stated that a widespread storm on 29 May through 1 June 1894, in the vicinity of Morrison, produced a flood that caused the loss of bridges, railroad tracks, houses, and destroyed the highway in the canyon. Descriptions of some more notable flood events follow:

July 24, 1896
Intense rainfall on 24 July 1896 centered on Cub Creek, a tributary of Bear Creek near Evergreen. "Without a moment's warning the largest flood that ever came down Bear Creek struck Morrison about 8 o'clock tonight (July 24), sweeping everything in its path ... although the water came down through the town nearly 3 feet deep in the main street, the buildings in the business section all withstood it." Twenty-seven lives were lost in the flood (available records do not indicate where the deaths occurred) and severe damages were reported from Evergreen to the mouth of Bear Creek. No rainfall records of this flood are available. The peak flow on Bear Creek at the Morrison gaging station was estimated at 8,600 cfs, which is the flood of record for the gage. The most recent hydrologic studies indicate that this flood would have a one in 40 chance of occurring in any year. It is not known to what extent Mount Vernon Creek contributed to the Morrison flooding.
July 7-8, 1933
"Five persons known dead ... property damage of unestimated degree and nearly all the highway between Mt. Morrison and Idledale ruined, is the toll up to date of one of the most devastating floods last Friday afternoon (July 7) ever to visit the Bear Creek Watershed. ... A cloudburst at about 1 o'clock in the neighborhood of Idledale sent a wall of water down Saw Mill Gulch leading to Bear Creek, and another raging torrent down Vernon Creek. ... The Vernon Creek waters reached a height of 15 feet ... in the narrow passage between the business houses. … The Highway up beautiful Bear Creek Canyon between Mt. Morrison and Idledale is practically ruined." The peak discharge at Morrison was 8,000 cfs on Bear Creek and estimated as 1,500 cfs on Mount Vernon Creek.
August 9, 1934
The flood of 9 August 1934 in the Bear Creek basin was caused by cloudburst-type rainfall near Kittredge and at the head of Mount Vernon Creek. Six lives were lost and much property damage resulted. It was reported that Mount Vernon Creek ran higher than the previous year and much of the canyon roadway was destroyed. Damage to Morrison was reduced because the Bear Creek peak flow passed through the town before the Mount Vernon Creek high water arrived.
September 2-3, 1938
A widespread thunderstorm that began over the eastern slope of the Front Range on 30 August became most intense in the Morrison area on 2 September. An unofficial report stated that 7.9 inches fell just north of Morrison in six hours. The heaviest rainfall centered on the divide between Bear Creek and Mount Vernon Creek. The peak discharge on Bear Creek at Morrison above Mount Vernon Creek was 6,200 cfs. From post flood measurements the Mount Vernon Creek peak discharge was estimated at 9,230 cfs at a point 1/2 mile upstream from Morrison. From statements by local residents it appears that the peak discharge on Mount Vernon Creek reached Morrison at about 7 p.m., preceding that on Bear Creek by 1/2 hour. Six persons drowned when trapped in their automobile between Morrison and Kittredge. Damages in the basin were estimated at $450,000. If Morrison had not been warned, or if the flood had occurred late at night, the number of deaths would likely have been considerably higher. The photograph on page II-10 (Figure II-5) shows the relative level of this flood compared with the 100-year flood level. Note that this flood appears to have exceeded today’s regulatory flood elevation by as much as four feet.
August 24, 1946
On August 24, 1946, a heavy rain near Idledale caused Bear Creek to overflow. A Morrison woman was swept from her stranded car and drowned.
August 21, 1957
On the afternoon of 21 August 1957, thunderstorms occurred over the Bear Creek basin with heavy rain and hail beginning about 1 p.m. east of Squaw Pass and northwest of Evergreen. At most locations the rain stopped within an hour. The Mount Vernon Creek peak discharge at Morrison was estimated at 1,000 cfs at 2:30 p.m., and 1,640 cfs on Bear Creek at about 3 p.m. While most damages from Bear Creek occurred downstream of Morrison, Mount Vernon Creek left debris on the grounds of six or seven residences in Morrison, flooded a garage and a used car lot, and broke a water main. State Highway 8 at Morrison was closed upon warning of the flood. Later, portions of the highway were flooded by both streams.
July 25, 1965
On 23-24 July 1965, heavy rains over the headwaters of Bear Creek caused minor flooding throughout its length. Most damages occurred downstream of Morrison. A peak discharge of 1,030 cfs was measured for Bear Creek at Morrison on July 25, 1965.
May 7, 1969
Heavy rains from 4-8 May 1969 resulted in flooding in the Bear Creek basin with most damages occurring downstream from Morrison. A weather station at Morrison reported a total storm rainfall of 11.27 inches, with a maximum daily amount of 5.77 inches. Unofficial rainfall amounts in the basin varied from 6.7 inches to 11.8 inches during the 5-day storm period. The peak flow for Bear Creek at Morrison was 2,340 cfs on May 7, 1969. The following 1969 newspaper clipping also provides interesting accounts of the 1896 floods on Bear Creek at Morrison and Tucker Gulch at Golden.

Snowmelt Flood vs. Rainfall Flood Potential

To evaluate the snowmelt vs. rainfall flood potential at Morrison, Idledale, Kittredge and Evergreen, a discharge-frequency relationship at each location was established for both types of events. The Flood Insurance Study for Jefferson County, Colorado, originally prepared by PRC Engineering Consultants, Inc., provided the most complete hydrology study for Bear Creek. Included in that report was an estimate of discharge-frequency relationships for both snowmelt and rainfall floods at the Morrison stream gage. The study procedures were extended to develop the desired data and the results are summarized in Table II-2.

TABLE II-2
SUMMARY OF RAIN AND SNOWMELT FLOOD POTENTIAL

Location

Drainage
Area
(mi2)

Return Period & Discharge (cfs)
Rainfall Events

Return Period & Discharge (cfs)
Snowmelt Events

10-yr

50-yr

100-yr

500-yr

10-yr

50-yr

100-yr

500-yr

Morrison

164

2,180

8,130

13,490

39,850

600

1,080

1,310

1,940

Idledale

158

2,130

7,940

13,180

38,930

590

1,060

1,290

1,910

Kittredge

139

1,970

7,330

12,170

35,930

560

1,000

1,220

1,800

Evergreen

105

1,650

6,150

10,200

30,130

490

890

1,070

1,590

Anticipated Warning Times

Tables II-3 summarizes the estimated warning times that the automated stream gages provide to downstream areas. Lead times are given for three flood wave velocities of 12, 15 and 18 feet per second (fps). The value of 15 fps is assumed to be a reasonable estimate, but times based on 12 and 18 fps velocities are included to illustrate how flood travel times can vary. The lead times shown do not factor in the times required for data analysis and decision-making.

TABLE II-3
AUTOMATIC STREAM GAGES
LEAD TIME IN MINUTES FOR
INDICATED FLOOD VELOCITIES IN FPS

ALERT Gage & I.D. Numbers

MORRISON

IDLEDALE

KITTREDGE

EVERGREEN

FPS

12

15

18

12

15

18

12

15

18

12

15

18

Rosedale #2253

106

85

71

83

66

55

51

41

34

33

26

22

Cub Creek at Blue #2273

98

78

65

74

60

50

42

34

28

24

19

16

Evergreen Lake #2223

74

59

49

51

40

34

18

15

12

--

--

--

Bear Creek below Cub #2233

73

59

49

50

40

33

17

14

12

--

--

--

Cold Spring Gulch #2243

38

30

25

14

12

10

--

--

--

--

--

--

Red Rocks Park #2373

12

9

8

--

--

--

--

--

--

--

--

--

Flood Elevations in Morrison

The following photographs (Figures II-2 through II-5) were taken from the Morrison Flood Insurance Study. Each photo depicts the 100-year flood level at various locations in Morrison. Figure II-5 also shows the approximate high water mark for the 2-3 September 1938 flood.

Figure II-2

Park Avenue bridge over Bear Creek in Morrison

Figure II-3

Canon Street bridge over Bear Creek in Morrison

Figure II-4

Colorado Highway 74 bridge over Mount Vernon Creek in Morrison at confluence with Bear Creek

Figure II-5

Bear Creek flood levels on side wall of store in Morrison between Market Street and Mount Vernon Street, downstream of Mount Vernon Creek. Historic high water mark depicts level of 1938 flood.

References

the following documents contain additional hydrologic data, flood photos, historic flood descriptions, floodplain delineations and other pertinent information, and should be considered supplemental to this flood warning plan:

1. UDFCD, Flash Flood Warning Planning, Bear Creek, prepared by UDFCD, GRD Weather Center and Leonard Rice Consulting Water Engineers, January 1981.

2. UDFCD, Flood Hazard Area Delineation, Southern Jefferson County, prepared by PRC Engineering Consultants, Inc., December 1981.

3. UDFCD, Flood Hazard Area Delineation, Bear Creek, prepared by Gingery Associates, Inc., December 1979.

4. FEMA, Flood Insurance Study, Unincorporated Jefferson County, prepared by Gingery Associates (August 1983), revised July 4, 1989.

5. FEMA, Flood Insurance Study, Town of Morrison, prepared by PRC Engineering Consultants, Inc., June 1, 1982.