These are the archived records of the annual peak flows (annual series) for the analysis of the extreme flood events on the Blackfoot River. The records run from 1940 to 1994. Some records that you download may include all peak flows above a certain base flow - select and use only the peak flows for each year, discarding the rest for this analysis. This analysis will concentrate on extreme flood events and their return intervals.
| DATE YR.MO.DAY | FLOW CUBIC FEET/SEC |
| 1940.05.12 | 3800 |
| 1941.06.02 | 1940 |
| 1942.05.26 | 6900 |
| 1943.06.18 | 9750 |
| 1944.05.19 | 4280 |
| 1945.06.04 | 5540 |
| 1946.05.29 | 6540 |
| 1947.05.09 | 12800 |
| 1948.05.22 | 16300 |
| 1949.05.16 | 8840 |
| 1950.06.21 | 9400 |
| 1951.05.13 | 11000 |
| 1952.04.29 | 8820 |
| 1953.06.04 | 18300 |
| 1954.05.21 | 13300 |
| 1955.06.15 | 7060 |
| 1956.05.22 | 12100 |
| 1957.05.21 | 10100 |
| 1958.05.23 | 9110 |
| 1959.06.07 | 12200 |
| 1960.06.04 | 6880 |
| 1961.05.27 | 10200 |
| 1962.05.25 | 8790 |
| 1963.06.01 | 5060 |
| 1964.06.10 | 19200 |
| 1965.06.13 | 9960 |
| 1966.05.11 | 6420 |
| 1967.06.03 | 11900 |
| 1968.06.04 | 5960 |
| 1969.05.15 | 8340 |
| 1970.05.27 | 10900 |
| 1971.05.14 | 12100 |
| 1972.06.02 | 15700 |
| 1974.06.17 | 11900 |
| 1975.06.20 | 18100 |
| 1976.05.15 | 14000 |
| 1977.05.03 | 2190 |
| 1978.06.09 | 8390 |
| 1979.05.27 | 10800 |
| 1980.05.27 | 9110 |
| 1981.05.24 | 12400 |
| 1982.06.17 | 10200 |
| 1983.05.30 | 7760 |
| 1984.05.31 | 6610 |
| 1985.05.25 | 6590 |
| 1986.05.30 | 7300 |
| 1987.05.01 | 4850 |
| 1988.05.14 | 3640 |
| 1989.05.11 | 10300 |
| 1990.06.01 | 7200 |
| 1991.05.20 | 9290 |
| 1992.05.09 | 4210 |
| 1993.05.16 | 7300 |
| 1994.04.25 | 5910 |
Open the downloaded peak flow records in a spreadsheet. Arrange the information (as you see below) so that you have columns for date of peak discharge and for amount of discharge.
Sort the data in your spreadsheet in descending order, so that the largest annual peak flow is first and the smallest is last on your list. Insert a column numbered from 1 to however many years of record you have downloaded. This column ranks the annual peak flows from the largest (ranked 1) to the smallest. Each annual peak flow now has an "order number." Hydrologists have developed a relationship between the rank of an annual peak flood, the number of years of record available, and the return period* of a particular peak flood:
R = (n+1)/m
where "R" is the return period or recurrence interval in years, "n" is the number of years of record, and "m" is the rank of the peak flow or the order number ("m" equals 1 for the largest annual flow, and "m" equals "n" for the smallest annual flow).
Insert another column in your spreadsheet that calculates "R," the return period for each annual peak flow using this formula.
*It is important to remember that the return period is not a guarantee that a peak flow of a particular size will occur every so many years, but a statement about the probability of a peak flow of a certain size occurring in any one year.
TABLE 2: 1940-1996 - RETURN PERIOD AND MAXIMUM YEARLY FLOW, BLACKFOOT RIVER AT BONNER, MONTANA
| DATE | RETURN PERIOD | DISCHARGE |
| YR.M.D | (R = (n+1)/m) | CFS |
| 1964.06.10 | 55.0 | 19200 |
| 1953.06.04 | 27.5 | 18300 |
| 1975.06.20 | 18.3 | 18100 |
| 1948.05.22 | 13.8 | 16300 |
| 1972.06.02 | 11.0 | 15700 |
| 1976.05.15 | 9.2 | 14000 |
| 1954.05.21 | 7.9 | 13300 |
| 1947.05.09 | 6.9 | 12800 |
| 1981.05.24 | 6.1 | 12400 |
| 1959.06.07 | 5.5 | 12200 |
| 1956.05.22 | 5.0 | 12100 |
| 1971.05.14 | 4.6 | 12100 |
| 1967.06.03 | 4.2 | 11900 |
| 1974.06.17 | 3.9 | 11900 |
| 1951.05.13 | 3.7 | 11000 |
| 1970.05.27 | 3.4 | 10900 |
| 1979.05.27 | 3.2 | 10800 |
| 1989.05.11 | 3.1 | 10300 |
| 1961.05.27 | 2.9 | 10200 |
| 1982.06.17 | 2.8 | 10200 |
| 1957.05.21 | 2.6 | 10100 |
| 1965.06.13 | 2.5 | 9960 |
| 1943.06.18 | 2.4 | 9750 |
| 1950.06.21 | 2.3 | 9400 |
| 1991.05.20 | 2.2 | 9290 |
| 1958.05.23 | 2.1 | 9110 |
| 1980.05.27 | 2.0 | 9110 |
| 1949.05.16 | 2.0 | 8840 |
| 1952.04.29 | 1.9 | 8820 |
| 1962.05.25 | 1.8 | 8790 |
| 1978.06.09 | 1.8 |