Snow Melt and Its Effect on Flood Runoff of the Ohio River at Pittsburgh, Pa. R. J. MACCONNELL U. S. Weather Bureau, Pittsburgh, Pa. URING THE 1944-45 winter season, a and 80 miles wide. (See FIG. 1.) The range D near record accumulation of snow was in elevation above mean sea level in the present in the Ohio River drainage Allegheny is from 750 to about 3,000 feet above Pittsburgh, Pa. Similar snow accu- mulations existed in the 1909-10 and 1935-36 seasons. However, there is a wide divergence in the maximum flood peaks reached in the Ohio River at Pittsburgh fol- lowing the melting of the snow cover during these 3 seasons; namely, 26.0 feet in 1910, 46.0 feet in 1936, and 33.4 feet in 1945. This paper is presented to show the part snow melt plays in floods in the Ohio Basin at and above Pittsburgh. Snow melt, as a factor of flood runoff, is generally of sec- ondary importance in this area. The snow normally begins to accumulate in December and continues until March. There are inter- mittent periods of freezing and thawing with a subsequent increase and depletion in the snow cover and runoff. PHYSICAL FACTORS The watershed above Pittsburgh is com- posed of the Allegheny and Monongahela Rivers which have drainage areas of 11,733 and 7,384 square miles, respectively. The Allegheny River extends from approximately latitude 40°30' to 42°30' and the Mononga- hela from 38°30' to 40°30'. The Allegheny Basin is 160 miles long and 130 miles wide. FIGURE 1. Drainage Basin of the Upper Ohio The Monongahela Basin is 139 miles long River Above Pittsburgh, Pa. FIGURE 2. Mass-Curves of Degree-Days Above 32 Degrees, 1944-45 Unauthenticated | Downloaded 09/24/21 03:22 PM UTC with an average of 1,600 feet. The Monon- tions of the Cheat, Youghiogheny, Kiskimi- gahela ranges from 750 to 4,800 feet with an netas and lower Allegheny, and finally the average elevation of about 1,700 feet. The higher elevations of the Monongahela and physiographic features of the basin have a upper Allegheny (FIG. 1). This usually marked effect on the distribution of the gives a progressive southwest to northeast snowfall and snow melt. depletion of the snow cover. The melting The orientation of the basin is an impor- usually causes rather high stages in the tant factor in the snow melt in that the upper reaches of the tributaries. As the seasonal progression of temperature is from crests move downstream they are so modified the upper to lower Monongahela, to the lower (unless the melting is accompanied by heavy Allegheny, then the upper Allegheny water- rains) that by the time the crests reach sheds. As shown in FIG. 2, the degree-day Pittsburgh, the stages are greatly reduced. accumulation (32° base) is greatest in the The daily variation of the temperatures is Monongahela (Elkins, W. Ya.) and decreases also effective in modifying snow melt runoff northward; also the accumulation is earlier (see FIG. 3), in that there are alternate at Elkins, W. Ya. The snow melt first occurs periods of freezing and thawing. The tem- on the West Fork River, then the lower eleva- peratures usually have a mean daily range FIGURE 3. Daily Maximum and Minimum Temperatures, 1944-45 Unauthenticated | Downloaded 09/24/21 03:22 PM UTC of from 10° to 30° over most of the basin, ANALYSIS OF 1944-45 SEASON but decreasing northward. If heavy rains The first heavy snowfall during the occur during periods of melting, the snow melt is a contributing factor to the flood 1944-45 season occurred on December 11 and runoff, otherwise a large part of the snow 12. This snow was general over the basin melt is evaporated into the air and ab- and averaged about 15 inches with an aver- sorbed by the soil. The ground condition age water content of 1.5 inches. Light to during the winter period also is a factor in moderate snow fell intermittently during the the character of the flood runoff. If the season and the total snowfall reached a ground is not frozen when the snow falls, record in most parts of the basin and varied the runoff is modified by percolation; how- from 32 inches in the lower Monongahela ever, if the ground is frozen prior to major to 132 inches in the headwaters of the snows, the runoff is thereby aggravated. Youghiogheny and Kiskiminetas. There FIGURE 4. Stage Hydrographs, 1944-45 Unauthenticated | Downloaded 09/24/21 03:22 PM UTC was a gradual increase in the density of the tribute to flood run-off. The second rise in snow until December 23 when warmer February occurred on the 23rd. This was weather set in causing a rise in the upper caused by moderate rain and snow melt from Monongahela River. The temperatures rose the Cheat, Youghiogheny, Kiskiminetas and an average of about 30 degrees over the lower Allegheny Rivers. All crests were Monongahela Basin with an average maxi- below flood stage. Maximum temperature mum of about 55 degrees. This was accom- during this period averaged about 40°. panied by moderate rain causing rises in The first flood of the season at Pittsburgh all the rivers, with the major runoff from occurred on February 27th with the Ohio the Kiskiminetas and Monongahela Basins. River reaching a crest of 27.7 feet, 2.7 feet Flood stages were reached on the West Fork above flood stage. At the beginning of this River, one of the principal tributaries of the flood, the snow had disappeared from most Monongahela River. As the flood moved of the Monongahela and lower Allegheny. downstream, these stages were modified and Snow melt was a minor factor in this flood no flood stages were reported below Clarks- as the snow depths were about the same burg, W. Ya. following the flood. The Ohio River crested The snow cover, by the end of December, above flood stage again on March 4th at had been depleted over the lower elevations 26.9 feet. This rise was primarily from rain- of the Monongahela but had increased over fall, as the only significant snow cover was the rest of the watershed. A snow survey on the upper Allegheny and the tempera- made on December 29 showed the water con- tures over this part of the basin remained tent to be from 2 to 3 inches. There was near freezing. The maximum flood of the a gradual increase in the snow cover at eleva- season occurred on March 7th with a crest tions above 1500 feet from January 10 until of 33.4 feet for the Ohio River at Pittsburgh. February 15. The maximum snow cover This was the twenty-second highest flood of occurred about February 1 and, from the record and the sixteenth highest in the past snow survey made at that time, the water 100 years. Snow melt was a factor, but content of snow mantle varied from 3 to 8 secondary. The snow cover remaining on inches over the Allegheny Basin, averaging the upper Allegheny did not melt until fol- about 4 inches, and varied from a trace to lowing the flood of the 7th. The upper 4 inches on the Monongahela, averaging Allegheny, above Warren, Pa., was above about 2 inches. flood stage at several places from March 18 Streamflow at Pittsburgh reached a mini- to 25. However, no flood stages were mum for the season on February 6th. After reached on the lower river. The highest this date there was a gradual increase in the stage at Pittsburgh during this period was runoff, the first significant rise occurring 24.1 feet on the 22nd. The maximum rise on February 17th. (See FIG. 4.) This rise for the season from snow melt alone would was the result of snow melt, owing to tem- probably have been under flood stage. peratures gradually increasing over the basin. The run-off from the 6th to 17th of COMPARISON OF DIFFERENT SEASONS February was from melting snow and mod- erate precipitation, mostly from the Monon- The variation of floods at Pittsburgh dur- gahela and lower Allegheny Basins. A snow ing the seasons with heavy snow cover is survey made on February 17th showed that shown by comparing the stage hydrographs the snow cover had receded from the lower of 1910, 1936, and 1945. (See FIG. 5.) basin, no snow being reported below 1,000 Two floods occurred during the 1909-10 sea- feet. The snow cover on the upper Cheat son: Ohio River at Pittsburgh, Pa., reached Basin had a water content of about 2 inches; a crest of 26.0 feet on January 19 and 25.2 upper Youghiogheny and Kiskiminetas about feet on March 1. The precipitation for the 3 inches; Allegheny, above Parkers Landing, season was below normal. The maximum about 4 inches. About 40 per cent of the snow on the ground occurred on February basin was covered with snow that would con- 13, and varied from 8 to 48 inches. Unauthenticated | Downloaded 09/24/21 03:22 PM UTC FIGURE 5. Stage Hydrographs, Ohio River at Pittsburgh, Pa. Four floods were recorded during the Three floods were observed during the 1935-36 season with crests of 29.2 feet on 1944-45 season with crests of 27.7 feet on February 28; 25.0 feet on March 13; 46.0 February 28; 26.9 feet on March 4 and feet on March 18, and 30.6 feet on March 33.4 feet on March 7.