PLAIN INFORMATION

BIG THOMPSON RIVER LOVELAND TO THE LARIMER — WELD COUNTY LINE, COLORADO ; LITTLE THOMPSON RIVER BOULDER AND LARIMER COUNTIES NEAR BERTHOUD, COLORADO 1 y',

TC 423 .C6 PREPARED FOR B54 LARIMER COUNTY, BOULDER COUNTY 19 7 7 LARIMER - WELD REGIONAL COUNCIL OF GOVERNMENTS AND COLORADO WATER CONSERVATION BOARD BY DEPARTMENT OF THE ARMY, OM^IHA DISTRICT, CORPS OF ENGINEERS, 68102 '• * #

LIBRAR í SEP 19 1978

Bureau oí Reclamation Der»-' 'orado

/

Mi BUREAU OF RECLAMATION DENVER LIBRARY 92026874 CONTENTS *1202bfl74 Page Preface...... I Background Information ...... I Sett I em e n t ...... 1 The Stream and its V a l l e y ...... 2 Development on the Flood Plain ...... 10 Flood Situation ...... II Sources of Data and Records ...... 11 Flood Season and Flood Characteristics ...... 12 Factors Affecting Flooding and Its Impact ...... 12 Obstructions to floodflows...... 12 Flood damage reduction measures ...... 13 Other factors and their impacts ...... 14 Flood warning and forecasting ...... 14 Flood fighting and emergency evacuation plans 15 Material storage on the flood plain ...... 15 Past Floods ...... 16 Summary of Historical Floods ...... 16 Flood Records ...... 18 Flood Descriptions ...... 19 June 1921 ...... 19 June 1923 ...... 20 September 1938 ...... 20 June 1949 ...... 20 August 1951 ...... 21 Future Floods ...... 22 Frequency ...... 23 Hazards of Large Fl oo ds...... 24 Flooded areas and flood damages ...... 24 Obstructions to floodflows ...... 27 Velocities of flow ...... 28 Rate of rise and duration of flooding ...... 28 GIossary of Terms ...... 29 I TABLES

Tab le Page 1 Big Thompson River Basin Vicinity Climatological Data ...... 3 2 Stream Gag i ng Data ...... II 3 Annual Peak Discharges for the Big Thompson River Near D r a k e ...... 16 4 Annual Peak Discharges for the Big Thompson River Near La Salle ...... 17 5 Annual Peak Discharges for the Little Thompson River Near Berthoud ...... 18 6 Summary of Pertinent Discharges for the Big Thompson River Basin ...... 23 7 Flood Plain Reference Data, Big Thompson River Loveland to the Larimer-Weld County Line, Colorado...... 32 8 Flood Plain Reference Data, Little Thompson River Boulder and Larimer Counties Near Berthoud, Co I or a d o ...... 36

PHOTOGRAPHS

Figure Page 1 Big Thompson River. Looking downstream from County Road 9E (reference point I) ...... 5 2 Big Thompson River. Looking downstream from U. S. Highway 87 (reference point 22) ...... 5 3 Big Thompson River. Looking upstream from County Road 3S (reference point 37) ...... 6 4 Big Thompson River. Looking upstream from the Larimer-Weld County Line (reference point 50) .... 6 5 Little Thompson River. Looking upstream from County Road 4 (reference point 59) ...... 7 6 Little Thompson River. Looking downstream from County Road 4 (reference point 59) ...... 7 7 Little Thompson River. Looking upstream from U. S. Highway 287 (reference point 69) ...... 8

ii CONTENTS (continued)

PHOTOGRAPHS (contInued)

-v ÜME* Page 8 Little Thompson River. Looking downstream from U. S. Highway 287 (reference point 69)...... 8 9 Little Thompson River. Looking upstream from County Road 6C (reference point 99).... . 9 I® Little Thompson River. Looking downstream from County Road 6C (reference point 99)...... 9 Little Thompson River. Looking upstream from the Larimer—Weld County Line (reference point (103)...... ,0

PLATES T-itle Plate Number Basin Map ...... I Plate Index Map ...... 2 Flooded Areas ...... 3_8 Prof i I es ...... 9-23 Cross Sections ...... 24-28

iii BIG THOMPSON RIVER BASIN

LITTLE THOMPSON RIVER BASIN

BOULDER CO. WELD CO

BASIN BOUNDARY AT DOWNSTREAM LIMITS OF STUDY

BASIN MAP NO SCALE PLATE I PREFACE

This report describes the fload characteristics of the Big Thompson River and the. L ittle Thompson River in eastern Larimer County, Colorado, and the Little Thompson Riven in northern Boulder County, Colorado. The study reach o f th e Big Thompson Riven, extends from about 2 miles southeast of Loveland downstream to the Larimer- Weld County line. The Little Thompson Riven, study reach extends fAm the eastern face of the Rocky Mountains about 7.5 miles south­ w est o f Benthoud downstream to th e LanimeA-Weld County lin e . Land use along these stneams is primarily agricultural. This AepoAt was prepared fan the guidance of local offi- cials in planning the use and regulation of the flood plain. Two potential floods weAe used to AepAesent degAees of major flooding th a t may occjua in the future. These two floods, the 100-year and the 500-yean, one defined in the Glossany and should be given appro- pAiate consideration in planning for safety of development in the flood plain. The two potential floods one further defined by flooded aAea maps that show the appAoximate aaeas that would be inundated, flood profiles show the water depths Aelative to the stAeambed and flood elevations acAoss the width of the valley. Caoss se c tio n s aAe pAesented to indicate gAound level acAoss the valley and the over- lying flood depths. The flood profiles and flooded aAea data pAe­ sented aAe based on the existing conditions of the basin, stAeam, and valley when the AepoAt was pAepaAed. Possible future improve­ ments to contAol floods aAe not a considenation of this AepoAt. The information in this AepoAt does not imply any federal interest oa authority to zone oa Aegulate use of the flood plain ; this is a local Aesponsibility. The AepoAt provides a suitable basis for the adop­ tion of land use controls to guide flood plain development with consideration for environmental attributes, thereby preventing inten­ sification of loss problems. Because the report identifies flood problems, i t w ill stimulate the development of other flood damage reduction techniques such as flood control, removal of obstructions, and flood proofing; these techniques might be used in an overall flo od P lain Management I f PM) program. At the request of the former Larimer-Weld Regional Planning Commission, now the Larimer-Weld Regional Council of Governments, through the Colorado Water Conservation Board, this report was pre­ pared by the Omaha D istrict, Corps of Engineers, under the continuing authority provided in Section 206 of the 1960 flood Control Act {Public Law £6-645), as amended. The cooperation of local officials and public and private interests in providing assistance and information was most helpful. Distribution of the report to officials, agencies, and individuals concerned with planning in the area covered w ill be made by Larimer County and by Boulder County. The Corps of Engineers w ill provide interpretation and technical assistance, if requested, in the application of the report data. Other guidelines available from the Corps of Engineers are a pamphlet, "Guidelines for Reducing flood Damages", and a booklet, "Introduction to flood Proofing". i BACKGROUND INFORMATION

SETTLEMENT The settlement of Colorado essentially began in 1540 when Don Francesco De Coronado and his party entered the region. These Spaniards found many tribes of Indians. The Ute, Arapahoe, and Cheyenne Indian Tribes resided in the mountain areas and on the plains to the east. Later, in 1806 and in 1820, respectively, Zebulon Pike and Stephen H. Long explored the area, each discover­ ing a mountain peak that today bears their name. Fur traders and mountain men also explored the area in the early 1800’s contribut­ ing to the eventual settlement of the area. The search for wealth and status, however, brought on by the gold rush of 1858 contrib­ uted most to settlement of Colorado. This rush brought not only gold seekers but merchants, farmers, and ranchers who established permanent settlements.

The first permanent settler was Mariano Modena who set­ tled near present day Loveland in 1858. By I860, several families had settled in the Thompson valley primarily raising cattle and a few crops. In 1867, irrigation began with the construction of the first major ditch to divert water from the Big Thompson River. Railroad service to the area began in 1877 when the Colorado Central Railroad completed a rail line from Golden, Colorado to Cheyenne, Wyoming. The addition of railroad facilities added to the pros­ perity of the valley. By 1901, several manufacturing companies were operating in the area. These included a sugar beet processing factory, fruit and vegetable canning, a flour mill, a grain ele­ vator, a stone quarry, and a plaster mill.

The present day area in the vicinity of the Big and Little Thompson Rivers is primarily devoted to agriculture. However, due

I to the relative close proximity of the area to the Denver metro­ politan area, the Big and Little Thompson River valleys are exper­ iencing urbanization pressures.

THE STREAM AND ITS VALLEY The Big Thompson River basin drains an area of about 830 square miles in north central Colorado. Of these 830 square miles, approximately 610 square miles are mountainous and 220 square miles are plains. The basin is about 60 miles long and about 30 miles wide at its most extreme points. Elevations in the basin range from near 13,000 feet mean sea level (m.s.l.) near the Continental Divide to near 4700 feet m.s.l. at the basin's confluence with the South Platte River. Vegetation in the basin varies considerably relative to the basin topography. Forested areas and grasslands are typical of the mountainous region of the basin. In the plains region, the vegetation is typified by agricultural crops, grass­ lands, and intermittent stands of timber especially adjacent to streams. Climate in the basin is semi arid varying with the topog­ raphy. In general, warm summers and mild to cold winters prevail. Intense thunderstorms, sometimes of cloudburst intensity, can occur during the summer months. The following table gives climatological data for the Big Thompson River basin vicinity.

2 Table 1 . , Big Thompson River Basin Vicinity Climatological Data-^-'

Mean Annual July Mean January Mean Location Precipitation Temperature Temperature (inches) (°F) (°F)

Fort Col lins 14.94 70.8 26.8

2 miles East- Southeast of Post Office in Longmont 12.74 71.6 26.8

Gree1ey 12.20 73.3 24.5

Estes Park 15.87 62.2 27.1

— This data was compiled from National Weather Service records for 1941-1970.

The primary streams in the Big Thompson River basin are the Big Thompson River and the Little Thompson River. The Big Thompson River has its source in Rocky Mountain National Park near Milner Pass. From this area the river flows in a general easterly direction to its confluence with the South Platte River about 5 miles southwest of Greeley. The portion of the Big Thompson River studied in this report begins about 2 miles southeast of Loveland near the center of Section 20, T5N, R68W and ends about 5 miles downstream at the Larimer-Weld County line between Section 24, T5N, R68W and Section 19, T5N, R67W. The upstream study limit of this report is the downstream study limit of Flood Plain Informa­ tion. Big Thompson River. Loveland. Colorado, published by the Corps of Engineers in December 1971. Throughout this reach the channel width varies from 80 to 160 feet and averages 100 feet in width. The channel depth ranges from 4 to 9 feet and averages 6 feet. The streambed has an average slope of 9.5 feet per mile through the study reach. The channel is alluvial and meanders through a "u"-shaped valley scarred by considerable mining activity. 3 The Little Thompson River, a right-bank tributary of the Big Thompson River, drains an area of about 200 square miles. The river has its source at the eastern edge of Rocky Mountain National Park near Twin Sisters Peaks and flows easterly to join the Big Thompson River about 1 mile north of Mi I liken. Eleva­ tions in the basin range from about 11,000 feet m.s.l. at the headwaters to about 4725 feet m.s.l. at the Big Thompson River confluence.

The reach of the Little Thompson River studied in this report begins approximately on the section line between Sections 2 and 3, T3N, R70W and ends about 10 miles downstream at the Larimer-Weld County line between Section 24, T4N, R69W and Sec­ tion 19, T4N, R68W. The Little Thompson River throughout this reach is characterized by a meandering channel flowing through a "v"-shaped valley. Channel dimensions vary through the study reach. From reference points I to 46, the channel width varies from 60 to 160 feet and averages 100 feet; the depth ranges from 7 to 9 feet. From reference points 46 to 79, the channel width varies from 80 to 140 feet and averages 90 feet; the depth ranges from 4 to 9 feet. From reference points 79 to 104, the channel width varies from 60 to 130 feet and averages 90 feet; the depth ranges from 4 to 8 feet. The streambed has an average slope of 37 feet per mile from reference points I to 46, 25 feet per mile from reference points 46 to 79, and 17 feet per mile from refer­ ence points 79 to 104.

Figures I through 4 show existing conditions along the Big Thompson River study reach. Existing conditions along the Little Thompson River study reach are shown on figures 5 through II. These photographs were taken In December 1976 by personnel of the Colorado Water Conservation Board. 4 !

Figure I. Big Thompson River. Looking downstream from County Road 9E (reference point I).

Figure 2. Big Thompson River. Looking downstream from U. S. Highway 87 (reference point 22). 5 Figure 3. Big Thompson River. Looking upstream from County Road 3S (reference point 37).

Figure 4. Big Thompson River. Looking upstream from the Larimer-Weld County Line (reference point 50). 6 Figure 5. Little Thompson River. Looking upstream from County Road 4 (reference point 59).

Figure 6. Little Thompson River. Looking downstream from County Road 4 (reference point 59). 7 Figure 7. Little Thompson River. Looking upstream from U. S. Highway 287 (reference point 69).

Figure 8. Little Thompson River. Looking downstream from U. S. Highway 287 (reference point 69). 8 Figure 9. Little Thompson River. Looking upstream from County Road 6C (reference point 99).

rigure 10. Little Thompson River. Looking downstream from County Road 6C (reference point 99). 9 Figure II. Little Thompson River. Looking upstream from the Larimer-Weld County Line (reference point 103).

DEVELOPMENT ON THE FLOOD PLAIN Development on the flood plain within the study area is relatively sparse. The most common form of development is that associated with farmsteads and an agricultural area. At present, the more congested development associated with urban areas remains external to the flood plain.

10 FLOOD SITUATION

SOURCES OF DATA AND RECORDS Stream gaging records from the United States Geological Survey stream gages on the Big Thompson and Little Thompson Rivers were used in the hydrologic analysis for this study. The gage locations and pertinent data pertaining to them are shown in table 2.

Table 2 Stream Gaging Data

Approximate Stream Gage Location Drainaqe Area Gaging Record (sq. mi.)

Big Thompson At mouth of canyon, 304 Continuous since River 500 feet upstream 1887 except for from Handy Ditch periods between diversion dam and 1892 to 1895, 6 miles east of 1904 to 1926, Drake. 1933 to 1938, and 1949 to 1951.

Big Thompson On left bank 0.7 828 Continuous since River mile upstream from 1914 except for a highway bridge, 1.6 period from 1915 miles upstream from to 1927. mouth, and 4 miles west of La Sal le.

Little At mouth of canyon, 101 Continuous since Thompson River 7.5 miles southwest 1929 except for a of Berthoud. period between 1930 and 1947. (Dis­ continued in 1961.)

Information on past floods was obtained from Corps of Engineers flood records, Larimer County, the U. S. Geological Survey, the Colorado Water Conservation Board, and local newspapers.

II Cross section data and topographic mapping having a contour interval of 2 feet and a scale of I inch equals 200 feet were established by photogrammetric methods from flights made in April 1975. These data were furnished by Larimer County and Boulder County in cooperation with the Colorado Water Conservation Board. U. S. Geological Survey 7.5 minute quadrangle maps with 10 foot contour intervals at a scale of I inch equals 2,000 feet were also used in the study. Bridge and roadway survey data were provided by the Colorado Water Conservation Board.

FLOOD SEASON AND FLOOD CHARACTERISTICS The Big Thompson River basin is located within an area which can receive very intense rainfall of cloudburst magnitude. Floods have resulted from storms covering large areas receiving heavy general rainfall as well as from storms covering small areas but producing extremely intense rainfall. Floods generally occur in the period from May through July. Annual peak discharges in the study reach, however, have been experienced from March through September.

Floods in the Big Thompson River basin can have consid­ erably different characteristics depending upon the location within the basin. Floods in the mountainous region of the basin typically have rapid rises, high maximum discharges, and relatively short durations. Floods in the plains region, generally, are of lower peak discharge and have longer duration. Floods within the study reaches generally have a duration of about 1 day. Flood peaks from thunderstorms covering small areas are generally rapidly atten­ uated in the study reaches by valley storage of floodwaters.

FACTORS AFFECTING FLOODING AND ITS IMPACT Obstructions to floodflows - There are 6 roadways and 13 roadways which cross the Big Thompson River flood plain and

12 the Little Thompson River flood plain, respectively, as described in this report. Interstate Highway 25 presents the largest poten­ tial for obstructing floodflows on the Big Thompson River. County roads at reference points 47, 59, 82, and 103, the Colorado and Southern Railway, and U. S. Highway 287 present the largest flood- flow obstruction potential on the Little Thompson River.

Manmade objects, such as buildings, cars, and fences, as well as trees, brush, and other vegetation located on the flood plain, can also cause floodflow obstructions. These items, while remaining in place, not only tend to create higher stages on the flood plain because of reduced flow area and flow blockage but may also create higher localized velocities as the floodwaters flow around them. Flood stages may be lowered in a localized area, if the floodflows are of sufficient magnitude to dislodge these flow obstructions. The debris may then lodge at locations farther down­ stream, thus compounding problems. This floating debris commonly lodges against bridge piers and bridge decks; the flow area is then reduced or the channel is completely blocked. The blockage can produce severe stage and velocity increases, often accompanied by bridge damage or complete failure.

Ice effects have, historically, not been a problem on the Big Thompson and Little Thompson Rivers. Ice jams, however, could occur in the future, although the chance of occurrence of such a phenomenon is quite remote.

Flood damage reduction measures - Olympus Dam, constructed in 1949 by the Bureau of Reclamation, is located near Estes Park in the upper Big Thompson River basin. This structure, however, has negligible effect in reducing flood damages on the reach of the Big Thompson River studied in this report because of the relatively

13 large distance between the two locations. Flood damages are reduced to various extents, depending upon the degree of protec­ tion provided, by levees constructed by private interests along the Big Thompson River in the study reach. Unfortunately, the presence of levees such as these provide a false sense of security since they do not provide adequate protection against floods such as the 100-year flood.

Flood plain zoning can be an effective method for pre­ venting future damages. To date, flood plain zoning in Larimer and Boulder Counties has been instituted where flood plain informa­ tion is available. Any Federal related financial loans for build­ ing purposes in this special flood hazard area require flood insur­ ance on the property.

Other factors and their impacts - Flood damages are also possible in the event of large amounts of runoff from the numerous tributaries and small drainageways flowing into the Big Thompson and Little Thompson Rivers. Crops, roads, and farm buildings could receive some damage from these smalI streams. Although the study area is relatively clear of urban development, Larimer County has been experiencing residential growth in the vicinity of both the Little Thompson River valley and the Big Thompson River valley. The flood damage potential in the Big Thompson River basin can be reduced by implementing flood emergency plans and sound flood plain management concepts.

Flood warning and forecasting - The National Oceanic and Atmospheric Administration (NOAA), National Weather Service, sup­ plies weather forecasts at least twice daily for the area. Special forecasts of severe storms, flood warnings, and snowmelt advisaries of a general nature are also issued as required. Radar observation

14 of intense precipitation and projection of storm movement gives some lead time for flash flood warnings; this information is promptly disseminated to all news media and local officials by the best communications available.

Flood fighting and emergency evacuation plans - The Larimer County and Boulder County Civil Defense Offices coordinate disaster activities with the State Civil Defense Office. They feel that they have adequate organization to meet any emergency.

Material storage on the flood plain - Floatable manmade material stored on the flood plain is often washed away during floods with the potential of collecting at points downstream. The flood plain in the study reach is relatively clear of such materials. When floatable objects are stored on the flood plain, they should be secured in some fashion. Other floatable material, such as trees, branches, brush, and logs, is in evidence along the rivers. This material could form obstructions to floodflows, especially at bridges, in the event of a flood.

15 PAST FLOODS

SUMMARY OF HISTORICAL FLOODS Large flood discharges have occurred in the study reach of the Big Thompson River and the Little Thompson River. Tables 3, 4, and 5 show data collected at the gaging stations operated by the U. S. Geological Survey located at the mouth of the canyon east of Drake and 4 miles west of La Salle on the Big Thompson River and at the station located 7.5 miles southwest of Berthoud on the Little Thompson River.

Table 3 Annual Peak Discharges!/ for the Big Thompson River Near Drake£/

Date Discharge (c.f.s.) # 7 Jul 1906 6,000 * 1 Sep 1938 5,600 22 Jun 1941 4,690 7 Jun 1942 3,730 19 Jul 1945 7,600 21 Jun 1947 2,320 4 Jun 1949 3,330 # 3 Aug 1951 3,530 9 May 1957 2,040 16 Jun 1965 2,220 31 Jul 1976 31,200 (estimated)

1! Only those years having a peak discharge of 2,000 c.f.s. or greater are indicated. 2/ From U.S.G.S. Gaging Station at mouth of canyon, 500 feet up­ stream from Handy Ditch diversion dam and 6 miles east of Drake. * Not a continuous record. 16 .Table 4 Annual Peak Discharges— for the^Big Thompson River Near La Salle—''

Date Discharge (c.f.s.) #

4 Aug 1951 6,100 9 May 1957 2,460 16 May 1958 1,760 4 Jun 1961 2,050 18 Jun 1965 1,480 8 May 1969 2,480 27 Apr 1971 1,130 22 May 1973 1,280 1 Aug 1976 2,500 1 / Only those years having a peak discharge of 1,000 c.f.s. or greater are indicated. 2/ From U.S.G.S. Gaging Station on left bank 0.7 mile upstream from highway bridge, 1.6 miles upstream from mouth and 4 miles west of La Salle.

Not a continuous record.

17 .Table 5 Annual Peak Discharges— for the Little Thompson River Near Berthoud^/

___Date PIscharqe (c.f.s.) 10 Aug 1930 3,620 *

8 Jul 1947 1,360 6 Jun 1949 3,500 3 Aug 1951 2.380 9 May 1957 4,000 8 May 1958 1,470 23 Jun 1961 2.380 #

— Only those years having a peak discharge of 1,000 c.f.s. or greater are indicated. 2/ — From U.S.G.S. Gaging Station at mouth of canyon, 7.5 miles southwest of Berthoud.

* Not a continuous record.

FLOOD RECORDS Two U. S. Geological Survey stream gaging stations on the Big Thompson River currently record discharge data pertinent to the study reach. No U. S. Geological Survey stream gaging station is currently located on the Little Thompson River. Stream gages used in the study were described in table 2. Flood accounts are available from local newspapers, the Corps of Engineers, the Colorado Water Conservation Board, Larimer County, and the U. S. Geological Suryey.

18 FLOOD DESCRIPTIONS Little information is available relative to historic flooding on the Little Thompson River probably because of its rural nature which has resulted in little flood damage during flood events. Some flood accounts are, however, available on the Big Thompson River. The following accounts of flooding on the Big Thompson River are representative of typical floods for which information is available. Many of these accounts do not describe specifically accounts of flooding within the study reach of this report. They do, however, indicate that flood conditions existed upstream thereby implying that floodwaters would be passing through the study reach.

During widespread floods of May and June 1864, the low­ lands of the Big Thompson River valley were flooded, settlers' cabins were washed away, and farmlands damaged. According to THE LOVELAND REPORTER of 7 June 1894, the flood of 31 May - I June of that year destroyed a dam near the canyon northwest of Loveland and the river was "fully half a mile wide". On 7 July 1906, a dis­ charge of 6,000 c.f.s. was recorded at the Drake station. A bridge near Drake was washed out. The flood peak lasted only a "few hours". THE LOVELAND DAILY HERALD of I August 1919 described a cloudburst upstream of Drake on 31 July - "a 10-foot wall of water swept down through the . . . canyon." There was no mention of damage in the plains area.

June 1921. General rains from 2 through 7 June measured a total of 5.29 inches at Longmont and 3.02 inches at Fort Collins. Apparently, the mountain regions received less, as Estes Park reported 2.45 inches for the 6-day period with 1.2 inches received on 4 June. High water destroyed the recorder at the Drake Station so the time and rate of the peak flow was not recorded.

19 The Big Thompson River overflowed its banks and THE LYONS RECORDER of 9 June of that year related "at a point near the river about half a mile from Loveland, a house, with its occupants, was washed from its foundation and carried nearly half a mile downstream . . ff

June 1923. Rains in the Big Thompson basin during 7 through 10 June resulted in a total of 2.49 inches at Estes Park, 2.99 inches at Long’s Peak, and 2.80 inches at Waterdale. The heaviest rain came on 9 June at these stations with 1.45, 2.55, and 1.97 inches respectively. Flows past Drake of 3,590 c.f.s. were recorded at 6:00 P.M. on 9 June and again at 6:00 A.M. on 10 June. Probably greater flows occurred between those hours. According to the LOVELAND REPORTER-HERALD of 9 June 1923: "At four o ’clock (today) floodwaters from the Big Thompson . . . had reached West First Street in the city . . . the cement paving south of Loveland is covered with water to a depth of over a foot. The Estes Park road and the valley near the plaster mills are covered."

September 1938. On 2 September after heavy cloud­ bursts, the Big Thompson River valley was again flooded. The highway in the canyon was damaged and closed for repairs. Many bridges were destroyed and some homes were damaged. On I I and 12 September, heavy rains again caused serious flash flooding in the canyon and the valley. Serious damages to the agricultural crops resulted in the plains area.

June 1949. Heavy rains in the headwaters area of the Big Thompson River basin resulted in a peak discharge on 4 June of 3,330 c.f.s. at the Drake station and 7,750 c.f.s. at the Love­ land station. Rainfall amounts on 4 June were 2.75 inches at Estes

20 Park and 3.66 inches a+ Wa+erdale. Between 4 and 7 June, Estes Park received 3.92 inches of rainfall while Waterdale received 5.30 inches. Lowland areas west of Loveland were flooded and U. S. Highway 34 to Estes Park was damaged.

August 1951. On 2 and 3 August 1951, intense rains over much of the basin caused severe flooding along the Big Thompson River from Buckhorn Creek to the mouth. The peak dis­ charge at Drake was 3,530 c.f.s. on 3 August. At the river mouth the flow peaked at 6,100 c.f.s. on 4 August. Estes Park received 1.31 inches of rain during 2 and 3 August; Waterdale measured 4.64 inches, of which 4.18 inches fell on 2 August. On Buckhorn Creek, a dam containing about 1,000 acre-feet of stored water, failed at 10:00 P.M. on 3 August. The flood crest reached Love­ land at 11:30 P.M. and the peak there was estimated to be 22,000 c.f.s. The river was reported to be a mile wide in places. Num­ erous rural homes were flooded, many swept from their foundations. One mile of U. S. Highway 34 west of Loveland was washed away. Irrigation works, such as diversion dams and canals, were des­ troyed, crop loss was heavy, and there was much sediment and ero­ sion damage. The lives of four people were lost and many were left homeless. Total damages from the flood were estimated at $602,000.

21 FUTURE FLOODS

Floods of the same or larger magnitude than those that have occurred could occur in the future. To determine the flood potential of the study area, the 10-year, 50-year, 100-year, and 500-year floods were analyzed. The results of this analysis are presented in this report as a means of demonstrating the effects of large floods.

Discharge magnitudes for floods analyzed in this report were based upon an analysis of stream gaging data at the U.S.G.S. stream gages located near Drake and La Salle for the Big Thompson River and near Berthoud for the Little Thompson River. Informa­ tion on these stream gages was presented in table 2. Because there are no streamflow gaging records between the La Salle gage and the upstream limit of each study, synthetic discharge- probability relationships were developed for the intervening drainage area using unit hydrographs and flood routing techniques and rainfall probabi11 ties from U. S. Weather Bureau Technical Publication No. 40. The following table presents discharge data used in this report.

22 Table 6 Summary of Pertinent Discharges for the Big Thompson River Basin

10-Year 50-Year 100-Year 500-Year Dra i nage Discharge Discharge Discharge Discharge Location Area Peak Peak Peak Peak (sq. mi.) (c.f.s.) (c.f.s.) (c.f.s.) (c.f.s.)

BIG THOMPSON RIVER

Upstream limit 5,000 12,100 19,000 40,000 of study

1nterstate 4,300 8,800 11,500 21,000 Highway 25

Larlmer-Weld 3,600 7,600 10,000 18,500 County Line

LITTLE THOMPSON RIVER

Upstream limit 101 3,300 6,600 8,800 17,000 of study

Dry Creek 143 2,850 5,800 7,500 13,400

Larlmer-Weld 2,800 5,500 7,200 12,800 County Line

FREQUENCY The 500-year flood Is not the largest flood that can occur, but the probabll ity of larger floods Is remote. As can be seen from the gaging records for the Big Thompson and Little Thompson Rivers, discharges smaller than either the 100-year or 500-year floods are much more common. Large floods, however, can happen; this was clearly demonstrated by the Denver area floods of 1965, the June 1972 flood at Rapid City, South Dakota, and the August 1976 flood In the Big Thompson canyon.

Flooding can occur with relatively little discharge In the event of channel blockage caused by debris or ice. In these 23 situations, water surface elevations rise until relief is achieved by flows over the flood plain. Historical data, however, indicate that obstructions due to ice are not likely to occur in the study reach. The formation of obstructions at bridges in the study reach is, however, quite possible - especially due to debris accumulation.

HAZARDS OF LARGE FLOODS The extent of damage caused by any flood depends on the topography of the area flooded, depth and duration of flooding, velocity of flow, rate of rise, developments on the flood plain, amount of debris in the floodwater, and ice conditions, if ice is a problem in that particular area. A 100-year flood or a 500- year flood on the Big Thompson River or the Little Thompson River in the study reach would result in the inundation of and subsequent damage to primarily agricultural properties and associated devel­ opments as well as public utilities and public roadways. Deep floodwater flowing at high velocity and carrying floating debris would create conditions hazardous to persons and vehicles attempt­ ing to cross flooded areas. In general, floodwater 3 or more feet deep and flowing at a velocity of 3 or more feet per second could easily sweep adult persons off their feet, thus creating definite danger of injury or drowning. Rapidly rising and swiftly flowing floodwater may trap persons in homes that are ultimately destroyed or in vehicles that are ultimately submerged or floated. Isolation of areas by floodwaters could create hazards in terms of medical, fire, or law enforcement emergencies.

Flooded areas and flood damages - Water surface profiles for the floods studied in this report were developed using the backwater computer program HEC-2, "Water Surface Profiles", devel­ oped by the U. S. Army Corps of Engineers. The computations were 24 based on channel and flood plain conditions as represented by survey data gathered in April 1975 and as supplemented by later field investigations.

The profiles on plates 9 through 23 show the water sur­ face elevations and water depth of the 10-year, 50-year, 100-year, and 500-year floods relative to the streambed. In this particular case and throughout this study, the streambed is actually the chan­ nel low flow water surface elevation since the surveys were con­ ducted by aerial photogrammetric methods. The water surface ele­ vations of the 100-year and 500-year floods were used to determine flood limits on the cross sections and topographic i.iaps. The results are shown on the Flooded Areas, plates 3 through 8. An index to the Flooded Areas plates is on plate 2. Representative cross sections are illustrated on plates 24-28. The cross sections show ground elevations across the valley on both sides of the channel and the depths of overbank flooding. Reference points, coinciding with the locations of the cross sections and pertinent planimetric features are shown on the Flooded Areas and profile for correlation between the drawings. Tables 7 and 8 provide data at reference point locations, including distance along the main channel centerline, elevations of the streambed, and elevations of the 10-year, 50-year, 100-year, and 500-year floods.

The Flooded Areas show the area that would be flooded by the 100-year and 500-year floods. The flood limits were located at each cross section and the intervening flood outlines were drawn based upon detailed topographic mapping, engineering judgment, and field observations. It is, however, possible that more or less flooding should be shown on the Flooded Areas. For a specific situation, where more detailed accuracy of flooded area is required, the flood limits can be more accurately

25 established by determining the water surface elevation from the profile or reference tables and then locating that elevation by survey on the flood plain.

A reference line is shown on the Flooded Areas for both the Big Thompson River and for the Little Thompson River. This line is used for reference only and does not necessarily coincide with the existing stream channel even though it may be referred to as such in this report.

The flood elevations shown on the profile and in tables 7 and 8 apply laterally from the channel over most of the flood plain width. Road crossings and other topographic features can alter the lateral flood elevations. Depending on whether these features divert or block flows, the flood elevations at the edge of the flood plain may be higher or lower than at the channel.

Flood plain widths vary from one area to another through­ out the study reach. The 100-year flood plain width of the Big Thompson River varies from a minimum near 800 feet to a maximum near 4,000 feet with an average of 1,200 feet. Likewise, the 100- year flood plain width of the Little Thompson River varies from a minimum near 100 feet to a maximum near 900 feet with an average of 500 feet. In general, the flood plains of both the Big Thompson River and the Little Thompson River are of relatively uniform width.

The major portion of the 100-year flood plain of both the Big Thompson River and the Little Thompson River in the study reach is in agricultural use. Few buildings of any type exist on this flood plain. Those that do exist are primarily agriculturally or i ented. 26 Obstructions to floodflows - Several roadways cross the flood plains of the Big Thompson River and the Little Thompson River in the study reach. The physical characteristics of these roadways created conditions in the study which resulted in an increase in water surface elevations immediately upstream from these roadways ranging from 0 to 5 feet. The upstream water sur­ face elevations would increase drastically if the bridge area is substantially reduced by debris especially at those roadways having high embankments. These higher water surface elevations would flood more area than shown in this report and increase velocities through the unobstructed portions of the bridges.

The profiIes on piates 9 through 23 show the extent to which flood stages are increased at the bridges. An appreciable stage increase occurs on the Big Thompson River upstream from Interstate Highway 25 and a county road at reference point 49. Similarly, an appreciable stage increase occurs on the Little Thompson River upstream from county roads at reference points 27, 47, 59, 74, 82, 91, 99, and 103 as well as upstream from U. S. Highway 287 and the Colorado and Southern Railroad. All roadways in the Big Thompson River study reach will be overtopped by the 100-year flood at some point along the roadway except Interstate Highway 25 and the Frontage Road at reference point 22. Similarly, all roadways in the Little Thompson River study reach will be overtopped by the 100-year flood except the county road at refer­ ence points 47, 59, 74, and 82, U. S. Highway 287, and the Colorado and Southern Railway.

Obstructions such as trees and buildings do exist on the flood plain in the study reach. The obstructive effect is, how­ ever, relatively insignificant because of the sparsity in number.

27 Velocities of flow - Water velocities during floods depend upon such factors as size and shape of the cross sections, conditions of the stream, and bed slope; all of these factors vary on different streams and at different locations along the same stream. On the Big Thompson River in the study reach, the channel velocity for the 100-year flood, in general, ranges from 5 to 10 feet per second. The overbank velocity for the 100-year flood ranges, in general, from 2 to 4 feet per second. The chan­ nel velocity for the 100-year flood on the Little Thompson River in the study reach ranges, in general, from 6 to 12 feet per second. The overbank velocity for the 100-year flood ranges, in general, from 2 to 4 feet per second.

Rate of rise and duration of flooding - Rate of ri se and duration of flooding, for floods comparable to a 100-year or 500-year flood, can vary over a wide range. A flood of rainfall origin on the Big Thompson River at Loveland would peak approxi­ mately 8 hours from the time it begins to rise. This flood would have an approximate duration of 20 hours. The same type of flood on the Little Thompson River at the upstream limit would peak approximately 3 hours from the time it begins to rise. The flood would have an approximate duration of 10 hours.

28 GLOSSARY OF TERMS

Flood An overflow on lands not normally covered by water and that are used or usable by man. Floods have two essential char­ acteristics: (I) the inundation of land is temporary; and (2) the land is adjacent to and inundated by overflow from a river, stream, an ocean, a lake, or other body of standing water.

Normally a flood Is considered as any temporary rise in streamflow or stage, but not the ponding of surface water, that results In significant adverse effects in the vicinity. Adverse effects may include damages from overflow of land areas, tempor­ ary backwater effects in sewers and local drainage channels, creation of unsanitary conditions or other unfavorable situations by deposition of materials in stream channels during flood reces- sions> rise of ground water coincident with increased strëamflow, and other problems.

Flood Crest The maximum stage or elevation reached by the waters of a flood at a given location.

Flood Plain The relatively flat area or low lands adjoining the channel of a river, stream, water course, ocean, lake, or other body of standing water which has been or may be covered by flood- water.

Flood Profile A graph showing the relationship of water surface ele­ vation to location, the latter generally expressed as distancé above mouth for a stream of water flowing in an open channel.

29 It is generally drawn to show surface elevation for the crest of a specific flood, but may be prepared for conditions at a given time or stage.

Flood Stage The stage or elevation at which overflow of the natural banks of a stream or body of water begins in the reach or area in which the elevation is measured.

Head Loss The effect of obstructions, such as narrow bridge open­ ings or buildings, that limit the area through which water must flow, raising the surface of the water upstream from the obstruc­ tion.

Left Bank The bank on the left side of a river, stream, or water course looking downstream.

Reference Point A numbered point identifying a specific location used for correlating the data shown in various forms throughout the report.

Right Bank The bank on the right side of a river, stream, or water course looking downstream.

10-, 50-, 100-, and 500-Year Flood A flood having a 10, 2, I, or .2 percent probability, respectively, of occurrence in any year or an average frequency of occurrence in the order of once in 10, 50, 100, or 500 years, respectively. The flood may occur in any year. It Is based on

30 statistical analyses of streamfIow records and analyses of rain­ fall and runoff characteristics in the general region of the watershed.

Sheet Flow The water which is diverted from the main floodflow by obstructions and variances in topography. This water flows at shallow depths and normally at elevations different from the water flowing in the adjacent channel or on the adjacent flood plain. Due to the shallow depths and irregular flow patterns associated with this type of flow, computations to predict flow depth are considered impractical.

Top of Waterway This is the roof of the opening in a stream crossing through which water flows under normal conditions. It is the underside of the deck span - sometimes called "low steel", the roof of a box culvert, or the crown of an arched or circular culvert.

31

Big Thompson River Loveland to the Larlmer-Weld County Line, Colorado

Stream- 10-Year 50-Year 100-Year 500-Year Reference Distance Bed Flood Flood Flood F 1 ood Point In E 1evat i on Elevation Elevation E 1evat i on Elevation Identification Number Feet Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Upstream Limit of Study - County Road I 122,390 4865.0 D/S 4874.2 D/S 4876.4 D/S 4878.2 D/S 4880.7 la 122,380 4865.0 4874.2 4876.4 4878.2 4880.7 Cross Section I-I 2 121,590 4864.5 4872.0 4874.1 4877.0 4878.7 3 121,090 4863.5 4871.8 4874.1 4876.9 4878.4 4 120,520 4863.5 4871.3 4873.7 4876.5 4877.2 5 119,600 4864.0 4869.5 4871 .2 '4873.2 4875.8 6 118,510 4863.5 4868.4 4870.0 4871.4 4874.2 Diversion 7 117,650 4863.0 4865.3 4867.1 4868.0 4869.2 Structure O 117,600 4856.5 4863.5 4865.2 4867.6 4868.9 9 117,190 4853.5 4860.9 4862.7 4863.8 4866.1 10 116,310 4852.5 4858.8 4860.3 4861.3 4863.5 I I 115,540 4850.0 4857.9 4859.1 4860.0 4862.1 12 114,960 4849.0 4856.1 4857.8 4858.9 4860.8 Cross Section 2-2 13 113,990 4847.0 4854.4 4856.1 4856.9 4859.0 14 113,510 4845.5 4852.4 4854.5 4855.5 4857.0 15 112,710 4844.5 4850.3 4851.4 4852.1 4853.8 16 112,180 4841.5 4848.5 4849.7 4850.5 4852.5

1/ Low water elevation as determined by aerial photogrammetric methods

Table 7 (continued) Flood Plain Reference Data Big Thompson River Loveland to the Larimer-Weld County Line, Colorado

Streami- 10-Year 50-Year 100-Year 500-Year Reference Distance Bed F 1 ood Flood Flood Flood Point in Elevation Elevation Elevation Elevation Elevation Identification Number Feet Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. 17 111,760 4841.0 4846.0 4847.0 4848.2 4850.8 18 110,110 4836.5 4843.0 4845.0 4846.3 4849.2 19 109,510 4835.5 4842.4 4844.5 4845.5 4848.1 Interstate Highway 20 109,430 4835.0 U/S 4842.3 U/S 4844.2 U/S 4845.3 U/S 4847.8 25 (Southbound) D/S 4842.2 D/S 4844.1 D/S 4845.2 D/S 4847.7 Interstate Highway 21 109,350 4835.0 U/S 4842.2 U/S 4844.0 U/S 4845.2 U/S 4847.3 25 (Northbound) D/S 4842.1 D/S 4843.9 D/S 4845.1 D/S 4847.2 Frontage Road 22 109,270 4835.0 U/S 4842.1 U/S 4843.8 U/S 4845.0 U/S 4846.9 D/S 4842.0 D/S 4843.7 D/S 4844.9 D/S 4846.8 23 109,000 4832.0 4841.8 4843.6 4844.6 4846.3 24 108,700 4832.0 4841.6 4843.4 4844.2 4846.0 25 107,850 4830.5 4840.8 4842.2 4843.0 4844.7 26 106,760 4829.0 4837.2 4840.0 4840.8 4842.5 27 106,170 4827.5 4836.0 4837.8 4838.6 4840.6 28 105,400 4827.5 4835.3 4836.7 4837.3 4839.0 29 104,720 4827.0 4834.4 4835.1 4836.2 4837.6 30 104,020 4826.5 4833.5 4834.7 4835.2 4836.6 Cross Section 3-3 31 103,350 4826.0 4832.9 4833.8 4834.3 4835.5

1/ Low water elevation as determined by aerial photogrammetric methods lauit? / v u u ii i i iiutiu / Flood Plaln Reference Data Big Thompson RIver Loveland to the Larimer-Weld County Line, Colorado

Stream^ 10-Year 50-Year 100-Year 500-Year Reference Distance Bed Flood Flood Flood Flood Point i n Elevation E 1evat i on Elevation Elevation Elevation 1 dentification Number Feet Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L.

32 102,700 4825.5 4831.6 4832.5 4832.9 4834.0 33 102,300 4825.5 4831.0 4831 .8 .4832.1 4833.0 34 100,560 4822.5 4829.0 4829.6 4829.9 4830.9 35 99,800 4819.5 4826.7 4827.5 4828.1 4829.4 36 98,500 4817.0 4825.7 4826.9 4827.5 4828.7 36a 97,210 4817.0 4824.7 4825.7 4826.1 4826.9 County Road 37 97,200 4817.0 U/S 4824.7 U/S 4825.7 U/S 4826.1 U/S 4826.9 D/S 4824.7 D/S 4825.7 D/S 4826.1 D/S 4826.9 w 38 94,340 4813.5 4819.9 4821.3 4821.9 4823.4 39 93,910 4813.0 4819.3 4820.8 4821.4 4822.8 40 91,910 4809.5 4817.9 4819.2 4819.8 4821.2 41 91,050 4808.5 4817.2 4818.4 4818.9 4820.3 Cross Section 4-4 42 90,280 4808.0 4815.6 4817.0 4817.5 4819.0 43 89,440 4807.0 4814.5 4815.8 4816.3 4817.7 Diversion 44 88,680 4806.5 4812.6 4814.1 4814.6 4815.6 Structure 45 88,660 4805.5 481 1 .5 4813.5 4814.2 4815.3 46 88,265 4804.5 4810.9 4812.4 4813.0 4814.5 47 87,070 4802.0 4808.0 4809.3 4809.8 4811 .2

1/ Low water elevation as determined by aerial photogrammetric methods Table 7 (continued) Flood Plain Reference Data Big Thompson River Loveland to the Larimer-Weld County Line, Colorado

Stream^ 10-Year 50-Year 100-Year 500-Year Reference Distance Bed Flood Flood Flood Flood Point in Elevation Elevation Elevation Elevation E 1evat i on Identif ication Number Feet Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L.

48 86,240 4801.0 4807.8 4809.0 4809.4 4810.6 Downstream Limit 49 85,540 4800.5 U/S 4806.2 U/S 4807.6 U/S 4808.1 U/S 4809.6 of Study Larimer-Weld County Line (County Road)

04

1/ Low water elevation as determined by aerial photogrammetric methods. Table 8 Flood Plain Reference Data L it t le Thompson River Boulder and Larimer Counties Near Berthoud, Colorado

Stream-^ 10-Year 50-Year 100-Year 500-Year Reference Distance Bed Flood Flood Point Flood Flood in Elevation Elevation Elevation 1 dent if ication Number Feet Elevation Elevation Ft.-M .S.L. Ft.-M .S.L. Ft.-M .S.L. Ft.-M .S.L. Ft.-M .S L Upstream Limit of Study 1 136,900 5222.0 5228.0 5230.5 5231.7 5234.6 2 136,600 5220.5 5226.3 5227.7 5228.7 5231.2 3 135,680 5213.0 5220.2 5222.0 5222.8 5224.0 4 134,750 5206.5 5212.6 5215.5 5216.2 5217.5 D iversion 5 134,540 5204.0 5207.6 Structure 5210.0 521l.l 5213.3 6 134,500 5201.0 5206.5 5208.9 5209.8 5212.1 7 134,280 5199.5 5205.0 5206.9 5207.1 5209.4 County Road 8 133,700 5198.8 U/S 5201.0 U/S 5201.9 U/S 5202.5 U/S 5204.0 D/S 5201.0 D/S 5201.9 D/S 5202.5 D/S 5204.0 9 133,600 5195.0 5198.7 5200.2 5200.7 5202.2 10 132,680 5185.5 5191.0 5192.5 5193.6 5195.5 1 1 131,750 5178.0 5183.0 5184.5 5185.1 5186.7 12 130,540 5169.5 5175.7 5177.4 5178.2 5179.5 13 129,830 5165,5 5171.1 5173.2 5173.8 5174.9 14 128,650 5156.5 5161.1 5162.2 5162.9 5164.3 15 127,720 5147.0 5151.9 5153.8 5154.6 5156.2 16 127,260 5143.5 5149.2 5150.5 5151.1 5152.2

1/ Low water elevation as determined by aerial photogrammetric methods Table 8 (continued) Flood Plain Reference Data Little Thompson River Boulder and Larimer Counties Near Berthoud , Colorado

Stream-^ 10-Year 50-Year 100-Year 500-Year Reference Distance Bed Flood Flood Flood Flood Point in E 1evat i on Elevation Elevation Elevation Elevation Identification Number Feet Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L.

Diversion 17 126,600 5141.0 5144.6 5146.6 5147.3 5149.9 Structure 18 126,550 5136.5 5141.1 5143.2 5144.3 5146.9

. 19 125,800 5130.5 5136.8 5138.0 5138.9 5140.7 20 125,000 5124.0 5131.1 5134.2 5134.8 5136.1 21 124,050 5119.5 5124.8 5126.7 5127.7 5130.6 22 123,470 5115.0 5119.5 5121 .5 5122.3 5125.3 23 123,000 5110.0 5116.2 5117.8 5118.6 5120.3 Cross Section l-l 24 122,400 5106.0 5112.6 5114.1 5114.7 5116.5 25 121,760 5101.5 5107.5 5108.8 5109.3 5111.6 26 121,700 5101.0 5107.3 5108.5 5109.2 5110.6 County Road 27 121,160 5097.7 U/S 5103.3 U/S 5106.7 U/S 5107.3 U/S 5108.7 D/S 5102.8 D/S 5104.8 D/S 5105.6 D/S 5107.4 27a 121,120 5097.0 5102.8 5104.8 5105.6 5107.4 28 120,460 5093.5 5099.8 5100.8 5101.6 5102.8 29 119,700 5088.0 5094.0 5095.6 5096.3 5097.6 30 119,040 5083.3 5089.3 5090.7 5091.4 5092.7 County Road 31 118,990 5086.6 U/S 5088.1 U/S 5090.1 U/S 5091.0 U/S 5092.5 D/S 5087.5 D/S 5089.8 D/S 5090.7 D/S 5092.4

— Low water elevation as determined by aerial photogrammetric methods. Table 8 (continued) Flood Plain Reference Data Little Thompson River Boulder and Larimer Counties Near Berthoud, Colorado

Stream--^ 10-Year 50-Year 100-Year 500-Year Reference Distance Bed Flood Flood Flood Flood Point in Elevation Elevation Elevation Elevation Elevation Identification Number Feet Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L.

32 118,940 5081.0 5086.9 5089.5 5090.4 5092.3 33 118,630 5078.0 5083.9 5086.8 5088.1 5090.1 34 117,900 5073.0 5078.5 5080.3 5081.2 5083.5 35 117,280 5070.5 5076.7 5078.3 5079.2 5081.3 36 116,750 5065.5 5071.6 5074.0 5074.8 5076.8 37 116,080 5062.0 5068.5 5070.0 5070.7 5072.2 38 115,350 5062.0 5065.5 5066.3 5066.8 5067.9 Diversion 39 115,200 5060.0 5061.9 5064.9 5065.4 5066.5 oo Structure 40 115,150 5056.0 5060.5 5063.5 5062.7 5064.8 41 114,750 5053.6 5057.5 5059.2 5060.4 5061.9 42 114,100 5046.0 5052.6 5054.5 5055.3 5058.8 43 113,420 5043.5 5048.9 5050.3 5051 .0 5052.6 Cross Section 2-2 44 112,730 5041.5 5046.5 5048.0 5048.7 5050.5 45 111,780 5037.0 5040.9 5042.1 5042.8 5044.4 46 110,880 5028.5 5035.4 5037.6 5038.3 5041.8 46a 110,190 5025.6 5032.1 5034.0 5037.0 5041.1 County Road 47 110,150 5025.6 U/S 5032.1 U/S 5034.0 U/S 5037.0 U/S 5041.1 D/S 5031.6 D/S 5033.0 D/S 5034.3 D/S 5036.6

1/ Low water elevation as determined by aerial photogrammetric methods Table 8 (continued) Flood Plain Reference Data Little Thompson River Boulder and Larimer Counties Near Berthoud, Colorado

Stream-^ 10-Year 50-Year 100-Year 500-Year Reference D i stance Bed Flood Flood Flood Flood Point in Elevation Elevation E 1evat i on Elevation Elevation Identification Number Feet Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L.

48 110,090 5025.5 5031.5 5032.9 5034.2 5036.5 49 109,280 5022.0 5027.1 5028.8 5029.4 5030.8 Cross Section 3-3 50 108,630 5017.5 5023.5 5025.0 5025.7 5027.3 51 107,900 5013.5 5019.0 5020.5 5021.3 5022.9 52 107,200 5011.2 5018.9 5018.3 5019.0 5020.5 53 107,070 5011.1 5016.5 5018.0 5018.6 5020.0 54 106,970 5011.0 5015.8 5017.1 5017.9 5019.5 55 106,400 5007.5 5012.0 5013.3 5013.9 5015.6 vO 56 105,720 5005.0 5009.9 5011.2 5011 .9 5013.2 57 105,080 5002.0 5007.0 5009.2 5009.7 5011.0 58 104,475 4998.0 5004.9 5006.6 5008.2 5010.0 County Road 59 104,250 4997.5 U/S 5003.4 U/S 5005.8 U/S 5007.4 U/S 5009.6 D/S 5001.8 D/S 5003.3 D/S 5004.0 D/S 5005.5 60 104,200 4995.5 5000.9 5001.8 5003.3 5005.3 61 103,700 4992.8 4999.2 5000.9 5001.7 5004.5 62 103,300 4992.0 4998.0 5000.1 5001.2 5004.0 63 102,860 4991.0 4996.0 4997.0 4997.7 5000.8 64 102,600 4990.5 4995.7 4996.8 4997.5 5000.0

— Low water elevation as determined by aerial photogrammetric methods. Table 8 (continued) Flood Plain Reference Data L i t t l e Thompson R ive r Boulder and Larimer Counties Near Berthoud, Colorado

Stream-^ 10-Year 50-Year 100-Year 500-Year Reference D i stance Bed Flood Flood Flood Flood P o in t in E le v a tio n E le va tio n E le v a tio n E le v a tio n E le v a tio n Identification Number Feet F t.- M .S .L . F t.- M .S .L . F t.- M .S .L . F t.- M .S .L . F t.- M .S .L .

65 101,600 4986.5 4992.8 4994.2 4994.9 4996.0 66 101,000 4984.7 4990.5 4991.8 4992.4 4994.7 67 100,510 4983.0 4988.4 4990.2 4991.1 4993.9 68 100,200 4982.0 4987.8 4999.7 4990.7 4993.8 U. S. Highway 287 69 99,670 4980.5 U/S 4987.1 U/S 4988.1 U/S 4989.4 U/S 4993.0 D/S 4985.5 D/S 4987.0 D/S 4987.9 D/S 4990.2

70 98,870 4977.0 4981.5 4983.1 4983.9 4986.6 71 98,200 4975.0 4979.8 4981.8 4982.6 4985.8 72 97,700 4973.5 4979.0 4981.1 4982.1 4985.6 73 97,120 4972.0 4978.5 4980.7 4981.8 4985.2 73a 96.950 4971.7 4978.4 4980.7 4981.7 4984.1 County Road 74 96,915 4971.5 U/S 4978.4 U/S 4980.6 U/S 4981.6 U/S 4983.8 D/S 4977.1 D/S 4979.0 D/S 4979.8 D/S 4981.8

D iversion 75 96,550 4970.0 4974.5 4976.7 4977.3 4979.7 Stru ctu re 76 96,520 4967.0 4970.2 4973.0 4973.9 4976.4 77 96,150 4963.0 4968.5 4970.4 4971.3 4974.1 78 95.950 4962.5 4966.8 4968.5 4969.3 4972.6 79 95,400 4959.0 4965.2 4967.3 4968.1 4970.3

— Low water elevation as determined by aerial photogrammetric methods riooa riain Keterence uaTa Little Thompson River Boulder and Larimer Counties Near Berthoud. Colorado

Stream-^ 10-Year 50-Year 100-Year 500-Year Reference Distance Bed Flood Flood Flood Flood Point i n Elevation Elevation Elevation Elevation Elevation Identif¡cation Number Feet Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L.

Cross Section 4-4 80 94,560 4956.5 4962.4 4964.2 4965.8 4968.0 81 93,650 4953.0 4959.8 4962.7 4965.3 4967.4 81a 93,420 4953.0 4959.7 4962.4 4965.0 4967.1 County Road 82 93,400 4952.8 U/S 4959.6 U/S 4962.3 U/S 4964.9 U/S 4967.0 D/S 4958.7 D/S 4960.9 D/S 4961.8 D/S 4963.9 83 92,520 4950.5 4956.9 4959.0 4960.1 4962.8 Dry Creek 83a 92,070 4949.0 4955.4 4957.2 4958.2 4962.3 84 91,850 4948.0 4954.6 4956.4 4957.2 4961.9 85 91,180 4945.5 4952.5 4954.7 4955.5 4961.6 86 90,550 4943.5 4951.9 4954.5 4955.2 4961.6 87 89,900 4943.0 4951.3 4954.1 4954.8 4961.4 88 89,560 4942 .'0 4950.7 4953.9 4954.6 4961.4 County Road 89 89,520 4941.0 U/S 4950.9 U/S 4953.9 U/S 4954.4 U/S 4961.3 D/S 4950.2 D/S 4953.5 D/S 4954.2 D/S 4961.3 Colorado & 90 89,385 4940.4 U/S 4950.1 U/S 4953.4 U/S 4954.1 U/S 4961.3 Southern Railway D/S 4950.0 D/S 4953.3 D/S 4953.9 D/S 4957.6 County Road 91 89,295 4940.0 U/S 4948.9 U/S 4953.2 U/S 4953.4 U/S 4956.3 D/S 4948.4 D/S 4950.6 D/S 4951.6 D/S 4954.3

1/ Low water elevation as determined by aerial photoqrammetr¡c methods Table 8 (continued) Flood Plain Reference Data Little Thompson River Boulder and Larimer Counties Near Berthoud, Colorado

Stream-^ 10-Year 50-Year 100-Year 500-Year Reference Distance Bed Flood Flood Flood Flood Point i n Elevation Elevation Elevation Elevation Elevation Identification Number Feet Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L. Ft.-M.S.L.

92 88,890 4939.0 4946.7 4948.3 4949.0 4950.7 93 88,150 4936.5 4944.2 4946.0 4947.0 4949.1 94 87,420 4934.5 4942.1 4944.3 4945.5 4946.6 95 86,600 4932.0 4938.2 4940.0 4940.7 4944.0 96 85,650 4929.0 4934.3 4936.2 4937.4 4939.5 97 85,600 4927.5 4934.2 4936.2 4937.4 4939.4 98 84,920 4925.0 4933.1 4935.1 4936.5 4938.3 County Road 99 84,655 4925.0 U/S 4933.0 U/S 4935.0 U/S 4936.4 U/S 4938.2

N> D/S 4931.1 D/S 4934.1 D/S 4935.7 D/S 4937.4 99a 84,620 4924.9 4931.0 4934.0 4935.6 4937.3 100 84,040 4923.0 4931.0 4933.1 4935.6 4937.3 101 83,250 4921.2 4929.1 4931.1 4935.2 4936.6 102 83,100 4921.1 4929.0 4931.0 4935.2 4936.6 103 83,065 4921.0 U/S 4928.9 U/S 4930.9 U/S 4935.2 U/S 4936.6 D/S 4928.6 D/S 4930.5 D/S 4931.7 D/S 4933.6 Downstream Limit 104 83,030 4921.0 4928.5 4930.4 4931.3 4933.1 of Study (Larimer-Weld County Line)

1/ Low water elevation as determined by aerial photogrammetric methods N LOVELAND

A it

SOUTH PLATTE RIVER BASIN BIG THOMPSON AND LITTLE THOMPSON RIVERS PLATE INDEX MAP U.S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 2 LEGEND

~1 500 100 YEAR FLOOD \ YEAR ______J FLOOD

Channel and Valley Cross Section

Reference Point - Also locations of surveyed cross sections

NOTES: 1. For the location of this plate, see Plate Index Map (Plate 2). 2. For illustrated Cross Sections, see Plates 24 a 25. 3. For Profile, see Plates 9,10 a II. 4. For flood elevations at the reference points, see Table 7. 5. For Detailed Mapping see Big Thompson River Topographic Mapping, Larimer County, Colorado. SCALE IN FEET

SOUTH PLATTE RIVER BASIN LOVELAND TO THE LARIMER - WELD COUNTY LINE , COLORADO BIG THOMPSON RIVER FLOODED AREAS U.S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 3 LEGEND

500 100 YEAR FLOOD YEAR FLOOD

Channel and Valley Cross Section DOWNSTREAM LIMIT OF STUDY— Reference Point - Also locations of surveyed cross sections

NOTES: 1. For the location of this plate, see Plate Index Map (Plate 2).

2. For illustrated Cross Sections, see Plates 2 6 a 27.

3. For Profile , see Plates II,I2,I3& I4. 4. For flood elevations at the reference points , see Table 7. 5. For Detailed Mapping see Big Thompson River Topographic Mapping, Larimer County, «50 YEAR FLOODS: Colorado. SHEET FLOW —'" " SCALE IN FEET fOOO 0______IOOO 2000

lO CN a 5 0 0 YEAR Ü SEE NOTE 5 f LOOO SHEET FLOW " LARIMER SOUTH PLATTE RIVER BASIN LOVELAND TO THE LARIMER - WELD COUNTY LINE , COLORADO BIG THOMPSON RIVER FLOODED AREAS U S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE I977

PLATE 4 LEGEND

500 YEAR FLOOD

Channel and Valley Cross Section

Reference Point - Also locations of surveyed cross sections

NOTES: R 70 W LARIMER COUNTY 1. For the location of this plate, BOULDER COUNTYi see Plate Index Map (Plate 2). 2. For Profile, see Plates 15,16 8 17. THOMPSON 3. For flood elevations at the reference points , see Table 8. 4. For Detailed Mapping see Little Thompson River Topographic Mapping, Larimer County — Boulder County, Colorado. U.S.G.S. STREAM GAGING STATION

UPSTREAM LIMIT SCALE IN FEET 1000 0______1000 2000 OF STUDY PLATE 2 see NOTE 4 PLATE i

SOUTH PLATTE RIVER BASIN BOULDER AND LARIMER COUNTIES NEAR BERTHOUD,COLORADO LITTLE THOMPSON RIVER FLOODED AREAS U.S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 5 LEGEND

500 100 YEAR FLOOD YEAR FLOOD

Channel and Valley Cross Section

Reference Point - Also locations of surveyed cross sections

NOTES: I. For the location of this plate, see Plate Index Map (Plate 2).

?. For illustrated Cross Sections, see Plate 28. 3. For Profile , see Plates 17,18 819. 4. For flood elevations at the reference points, see Table 8. 5. For Detailed Mapping see Little Thompson River Topographic Mapping , Larimer County ■— Boulder County, Colorado. SCALE IN FEET IOOO 0 1000 2000

SOUTH PLATTE RIVER BASIN BOULDER AND LARIMER COUNTIES NEAR BERTHOUD,COLORADO LITTLE THOMPSON RIVER FLOODED AREAS U.S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 6 LEGEND

~ 1 500 100 YEAR FLOOD S YEAR ______\ FLOOD

Channel and Valley Cross Section

COUNTY ROAD\ 500 YEAR FLOOD Reference Point - Also locations of surveyed cross sections r FLOW

NOTES: 1. For the locatron of this plate, see Plate Index Map (Plate 2).

2. For illustrated Cross Sections, see Plate 28.

3. For Profile , see Plates 19,20 8 21. 4. For flood elevations at the reference points , see Table 8. 5. For Detailed Mapping see Little Thompson River Topographic Mapping, Larimer County, Colorado. SCALE IN FEET 1000 0______1000 2000

SOUTH PLATTE RIVER BASIN PLATE 4 BOULDER AND LARIMER COUNTIES NEAR BERTHOUD,COLORADO NOTE 5 LITTLE THOMPSON RIVER FLOODED AREAS U.S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 7 LEGEND

Channel and Valley Cross Section

500 YEAR FLOOD SHEET FLOW Reference Point - Also locations of surveyed cross sections

NOTES: 1. For the location of this plate, see Plate Index Map (Plate 2).

2. For illustrated Cross Sections, see Plate 28.

3. For Profile, see Plates 22 8 23. 4. For flood elevations at the reference points , see Table 8. 5. For Detailed Mapping see Little Thompson River Topographic Mapping, Larimer County, Colorado. SCALE IN FEET COUNTY ROAD logo______o______loop 2000 COLORADO S SOUTHERN RAILWAY------DOWNSTREAM LIMIT SOUTH PLATTE RIVER BASIN BOULDER AND LARIMER COUNTIES PLATE 6 OF STUDY NEAR BERTHOUD,COLORADO

PLATE 7 LITTLE THOMPSON RIVER FLOODED AREAS U.S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

P LA TE 8 LEGEND ------500 YEAR FLOOD ------100 YEAR FLOOD ------50 YEAR FLOOD ------10 YEAR FLOOD

Deck I Bridge Top of Waterway

^1 — Reference Point

NOTES : I. For location of Cross Sections, see Plate 3. 2. For illustrated Cross Sections, see Plate 2 4 . 3. For flood elevations at the reference points,see Table 7.

SOUTH PLATTE RIVER BASIN LOVELAND TO THE LARIMER - DISTANCE IN FEET UPSTREAM FROM MOUTH WELD COUNTY LINE , COLORADO BIG THOMPSON RIVER PROFILE U S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 9 4870

4865 LEGEND

------500 YEAR FLOOD 4860 ------100 YEAR FLOOD ------50 YEAR FLOOD ui 5 4855 ------10 YEAR FLOOD tu UJ u_ 4850 Deck Bridge 4845 Top of Water woy % U i w | — Reference Point 4840

NOTES: 4835 I. For location of Cross Sections, see Plate 3. 4830 2 For illustrated Cross Sections, see Plate 25.

4825 3. For flood elevations at the reference points,see Table 7.

4820 116,000 115,000 114,000 113,000 112,000 1,000 110,000

SOUTH PLATTE RIVER BASIN OISTANCE IN FEET UPSTREAM FROM MOUTH LOVELAND TO THE LARIMER - WELD COUNTY LINE , COLORADO BIG THOMPSON RIVER PROFILE U S ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 10 ELEVATION IN FEET - M S L oo r i - => 1 - i mm o ) o Z 3 □ x x x Z Z O

ITNE N ET PTEM RM MOUTH FROM UPSTREAM FEET IN DISTANCE CORPS OF ENGINEERS OMAHA, NEBRASKA OMAHA, ENGINEERS OF CORPS U.S. ARMY ENGINEER DISTRICT, OMAHA DISTRICT, ENGINEER ARMY U.S. I TOPO RIVER THOMPSON BIG ED ONY IE COLORADO , LINE COUNTY WELD OEAD O H LRMR - LARIMER THE TO LOVELAND OT PAT RVR BASIN RIVER PLATTE SOUTH LEGEND . o fod lvtos t the at elevations flood For .I OTES: NO 1 eeec Point Reference — ^1 eeec pit,e Tbe 7. Table points,see reference I PROFILE UE 1977 JUNE 0 ER FLOOD YEAR 10 FLOOD YEAR 50 - FLOOD YEAR 100 - FLOOD YEAR 500 “ - Bridge Deck o o Waterway of Top

PLATE

II ELEVATION IN FEET - M S I DIS AC I FE USRA FO MOUTH FROM UPSTREAM FEET IN TANCE CORPS OF ENGINEERS OMAHA, NEBRASKA OMAHA, ENGINEERS OF CORPS U.S ARMY ENGINEER DISTRICT, OMAHA DISTRICT, ENGINEER ARMY U.S I TOPO RIVER THOMPSON BIG WELD COUNTY LINE , COLORADO , LINE COUNTY WELD OEAD O H LRMR - LARIMER THE TO LOVELAND OT PAT RVR BASIN RIVER PLATTE SOUTH LEGEND >-»» K| —Rfrne Point K#|Reference *— , . o ilsrtd rs Sections, Cross illustrated For 2. . o fod lvtos t the at elevations For flood 3. Fr oain f rs Sections, Cross of location For . I NOTES’...... se Pae 4. ¡Plate see - 1 YA FLOOD YEAR 10 r — eeec pit,e Tbe 7. Table points,see reference 1 . e Pae 26. Plate see 0 YA FLOOD YEAR 500 PROFILE -' 0 ER FLOOD YEAR 50 ~ UE 1977 JUNE ■ Woforwoy Topof i oo ER FLOOD YEAR

LT 12 PLATE

ELEVATION IN FEET - M S L DIS AC I FE USRA FO MOUTH FROM UPSTREAM FEET IN TANCE CORPS OF ENGINEERS OMAHA, NEBRASKA NEBRASKA OMAHA, ENGINEERS OF CORPS U S. ARMY ENGINEER DISTRICT, OMAHA OMAHA DISTRICT, ENGINEER ARMY S.U I TOPO RIVER THOMPSON BIG ED ONY IE COLORADO , LINE COUNTY WELD OEAD O H LRMR - LARIMER THE TO LOVELAND SOUTH PLATTE RIVER BASIN BASIN RIVER PLATTE SOUTH LEGEND ------500 YA FLOOD YEAR 0 0 5 ------NOTES: Fr lo eeain a the at elevations flood For . I ---- 5 YA FLOOD YEAR 50 ------eeec Point Reference — H eeec pit,e Tbe 7. Table points,see reference PROFILE UE 1977 JUNE Bridge 0 ER FLOOD YEAR 10 FLOOD YEAR 100 D o o Waterway of Top eck LATE P 13 LEGEND

------500 YEAR FLOOD ------100 YEAR FLOOD ------50 YEAR FLOOD ------10 YEAR FLOOD

Deck Bridge Top of Waterway

H — Reference Point

NOTES: 1. For location of Cross Sections, see Plate 4. 2. For illustrated Cross Sections, see Plate 27. 3. For flood elevations at the reference points,see Table 7.

DISTANCE IN FEET UPSTREAM FROM MOUTH

SOUTH PLATTE RIVER BASIN LOVELAND TO THE LARIMER - WELD COUNTY LINE , COLORADO BIG THOMPSON RIVER PROFILE U S ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 14 LEGEND

------500 YEAR FLOOD ------100 YEAR FLOOD ------50 YEAR FLOOD ------10 YEAR FLOOD

Deck I Bridge Top of Waterway

M — Reference Point

NOTES: I . For flood elevations at the reference points,see Table 8.

SOUTH PLATTE RIVER BASIN BOULDER AND LARIMER COUNTIES NEAR BERTHOUD,COLORADO LITTLE THOMPSON RIVER

DISTANCE IN FEET UPSTREAM FROM MOUTH PROFILE U.S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 15 Ll I

LEGEND ------500 YEAR FLOOD ------100 YEAR FLOOD ------50 YEAR FLOOD ------10 YEAR FLOOD

ì d 2 ! Deck i- UJ Bridge UJ u. 2 Top of Waterway ^1 — Reference Point % UJ NOTES : I . For flood elevations at the reference points,see Table 6.

r-LATTE RIVER BASIN BOULDER AND LARIMER COUNTIES NEAR SERTh OUD,COLORADO LITTLE THOMPSON RIVER PROFILE U S, ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 16 ELEVATION IN FEET - M S L DIS AC I FE USRA FO MOUTH FROM UPSTREAM FEET IN TANCE CORPS OF ENGINEERS OMAHA, NEBRASKA NEBRASKA OMAHA, ENGINEERS OF CORPS ITE HMSN RIVER THOMPSON LITTLE U.S. ARMY ENGINEER DISTRICT, OMAHA OMAHA DISTRICT, ENGINEER ARMY U.S. BOULDER AND LARIM ER COUNTIES COUNTIES ER LARIM AND BOULDER ------LEGEND — ER BERTHOUD,COLORADO NEAR SOUTH PLATTE RIVER BASIN BASIN RIVER PLATTE SOUTH ■ --- 2. For illustrated Cross Sections, Sections, Cross illustrated For 2. . o fod lvtos t the at elevations flood For 3. 1 . For location of Cross Sections, Sections, Cross of location For 1. ES: S TE O N ---- 1 Rfrne Point Reference — ^1 - eeec pit,e Tbe 8. Table points,see reference e Pae 6. Plate see e Pae 28. Plate see 50 ER FLOOD YEAR 500 ■ - PROFILE Bridge 0 YA FLOOD YEAR 100 0 ER FLOOD YEAR 50 0 ER FLOOD YEAR 10 UE 1977 JUNE D o o Waterway of Top eck LT 17 PLATE LEGEND

■ 500 YEAR FLOOD ------100 YEAR FLOOD ------50 YEAR FLOOD «------10 YEAR FLOOD

Deck Bridge Top of Waterway

<^1 — Reference Point

NOTES :

I . For flood elevations at the reference points,see Table 8.

so u l»1 platte r iv e r b a s in BOULDER AND LARIMER COUNTIES NEAR BERTHOUD,COLORADO LITTLE THOMPSON RIVER PROFILE U S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977 DISTANCE IN FEET UPSTREAM FROM MOUTH

PLATE 18 LEGEND — ... 500 YEAR FLOOD 100 YEAR FLOOD ------50 YEAR FLOOD ------10 YEAR FLOOD

Deck I Bridge Top of Waterway

^1 — Reference Point

NOTES: I. For location of Cross Sections, see Plate 6. 2. For illustrated Cross Sections, see Plate 28. 3. For flood elevations at the reference points,see Table 8.

SOUTH PLATTE RIVER BASIN BOULDER AND LARIMER COUNTIES NEAR BERTHOUD,COLORADO LITTLE THOMPSON RIVER PROFILE

DISTANCE IN FEET UPSTREAM FROM MOUTH U.S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 19 /

LEGEND

— 500 YEAR FLOOD “ “ 100 YEAR FLOOD co 5 ------50 YEAR FLOOD u i ------10 YEAR FLOOD in u. Deck Z 0 Bridge 1 UJ I Top of Waterway

H — Reference Point

NOTES: I. For location of Cross Sections, see Plate 7 2. For illustrated Cross Sections, see Plate 28. 3. For flood elevations at the reference points,see Table 8.

SOUTH PLATTE RIVER BASIN BOULDER AND LARIMER COUNTIES NEAR BERTHOUD,COLORADO DISTANCE IN FEET UPSTREAM FROM MOUTH LITTLE THOMPSON RIVER PROFILE U.S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATF 5>n LEGEND — — 500 YEAR FLOOD — ------100 YEAR FLOOD ---- — ----- 50 YEAR FLOOD ------10 YEAR FLOOD

Deck IBridge Top of Waterway

H — Reference Point

N O T E S : I . For flood elevations at the reference points,see Table 8.

103,000 102,000 101,000 100,000 99,000 98,000 97,000 96,000 SOUTH PLATTE RIVER BASIN BOULDER AND LARIMER COUNTIES NEAR BERTHOUD,COLORADO DISTANCE IN FEET UPSTREAM FROM MOUTH LITTLE THOMPSON RIVER PROFILE U.S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 21 LEGEND

■ 500 YEAR FLOOD ------100 YEAR FLOOD ------50 YEAR FLOOD ------10 YEAR FLOOD

Deck I Bridge Top of Waterway

H — Reference Point

NOTES: I. For locotion of Cross Sections, see Plate 8. 2. For illustrated Cross Sections, see Plate 28. 3. For flood elevations at the reference points,see Table 6.

SOUTH PLATTE RIVER BASIN BOULDER AND LARIMER COUNTIES NEAR BERTHOUD,COLORADO LITTLE THOMPSON RIVER

DISTANCE IN FEET UPSTREAM FROM MOUTH PROFILE U.S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 2 2 LEGEND

------500 YEAR FLOOD ------100 YEAR FLOOD — ---- 50 YEAR FLOOD ------10 YEAR FLOOD

Deck Bridge Top of Waterway

— Reference Point

NOTES: I . For flood elevations at the reference points,see Table 8.

SOUTH PLATTE RIVER BASIN BOULDER AND LARIMER COUNTIES NEAR BERTHOUD,COLORADO LITTLE THOMPSON RIVER PROFILE U.S. ARMY ENGINEER DISTRICT, OMAHA

DISTANCE IN FEET UPSTREAM FROM MOUTH CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 23 /

LEGENO ------500 YEAR FLOOD ------100 YEAR FLOOD

NOTES: I. For location of Cross Sections, see Plate 3

400 800 1,200 1,600 2,000 2,400

DISTANCE IN FEET

SOUTH PLATTE RIVER BASIN CROSS SECTION IH LOVELAND TO THE LARIMER - WELD COUNTY LINE , COLORADO BIG THOMPSON RIVER CROSS SECTION U.S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 2 4 4890

4885

\ 4880

if) 5 I 4875

UJ

4870 LEGEND Z o --- 500 YEAR FLOOD K 4865 5 ------100 YEAR FLOOD

4860

4855

4850

4 0 0 800 1,200 1,600 DISTANCE IN FEET NOTES : I. For location of Cross Sections, see Plate 3 (/> S

§ ÜJ SOUTH PLATTE RIVER BASIN LOVELAND TO THE LARIMER - WELD COUNTY LINE , COLORADO BIG THOMPSON RIVER CROSS SECTION U S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA CROSS SECTION 2-2 JUNE 1977

PLATE 2 5 LEGENO

------5 0 0 YEAR FLOOD

-J if) ------100 YEAR FLOOD S I H UJ UJ u. z o % UJ —J UJ

NOTES: I. For location of Cross Sections, see Plate 4

DISTANCE IN FEET

SOUTH PLATTE RIVER BASIN CROSS SECTION 3-3 LOVELAND TO THE LARIMER- WELD COUNTY LINE , COLORADO BIG THOMPSON RIVER CROSS SECTION U.S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 2 6 4840

LEGEND UJ UJ 500 YEAR FLOOD z 100 YEAR FLOOD o

UJ5 UJ

0 400 800 1,200 1,600 2 ,0 0 0 2 ,4 0 0 2,800 NOTES: DISTANCE IN FEET I. For location of Cross Sections, see Plate 4 CROSS SECTION 4 -4

SOUTH PLATTE RIVER BASIN LOVELAND TO THE LARIMER - WELD COUNTY LINE , COLORADO BIG THOMPSON RIVER CROSS SECTION U S. ARMY ENGINEER DISTRICT, OMAHA CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 2 7 LEGEND ------500 YEAR FLOOD -----— 100 YEAR FLOOD

0 400 800 1,200

DISTANCE IN FEET

CROSS SECTION l-l CROSS SECTION 2 -2

NOTES: I. For location of Cross Sections, see Plates 6 ,7 & 8.

SOUTH PLATTE RIVER BASIN BOULDER AND LARIMER COUNTIES NEAR BERTHOUD,COLORADO LITTLE THOMPSON RIVER 0 4 0 0 8 0 0 1,200 0 400 800 CROSS SECTION DISTANCE IN FEET DISTANCE IN FEET U.S. ARMY ENGINEER DISTRICT, OMAHA CROSS SECTION 3 - 3 CROSS SECTION 4 -4 CORPS OF ENGINEERS OMAHA, NEBRASKA JUNE 1977

PLATE 28