Allegheny lies partly in western Pennsylvania and partly in southwestern New York. The total area of the drainage is 11,683 square miles, 9,754 of which are in Pennsylvania and 1,929 in New York. The principal tributaries of the Allegheny are the Clarion and Kiskiminetas Rivers, and Red Bank, French, Brokenstraw, and Oil Creeks.

The drainage basin is about 185 miles long. In width it varies from 126 miles near the New York-Pennsylvania State line, to 47 miles at a point 25 miles above the mouth.

TOPOGRAPHY

The greater part of the Allegheny drainage lies in a much dissected plateau, part of the Allegheny plateau, and has at present the aspect of high rolling hills. The northwestern section of the watershed has been glaciated, so that the hills in this section are lower and more rounded and the valleys are less narrow and V-shaped. The southeastern part of the basin lies in the Allegheny Mountains, a region characterized by extensive ridges, the result of folding of the plateau. Only a small part of the drainage area confined to alluvial flats along the streams can be considered physiographically as "plains."

The approximate area in each of the four physiographical divisions of the basin is indicated in the following: Mountains, 9.4 per cent; hills, 75.3 per cent; rolling, 14.9 per cent; plains, 0.4 per cent.

The elevation at the head of the valley is slightly in excess of 2,200 feet, and at the mouth of the river, 703 feet. The greatest land elevation is in the southeastern portion of the valley in the main ridge of the Allegheny Mountains, where an elevation of 3,000 feet is reached. The western rim of the basin is lower, having a mean elevation of 1,330 feet. The mean elevation of the eastern rim is 2,275 feet. The slopes are steep, with the exception of those in the glaciated section.

The stream flows comparatively close to the western rim of the basin, leaving the greatest drainage area east of the main river, so that the eastern tributaries are much longer and much greater in number. This gives the valley in general a westerly exposure.

The streams in the northwestern, glaciated portion of the drainage are sluggish because they flow through buried valleys. The other tributaries to the main stream flow swiftly through narrow V-shaped valleys with short, steep grades and rocky beds. The upper reaches of the main stream are similar to the tributaries. The lower river from Pittsburgh north for about 50 miles has been slack watered by a series of six transportation dams.

Swamps are found in the glaciated section but not elsewhere. This is also true of lakes, of which there are about 15 in the drainage area. Chautauqua Lake in New York is the largest and Conneaut Lake in Pennsylvania is next in size. About 21 possible reservoir sites have already been surveyed on the main river and its tributaries. These combined would have a storage capacity of 49,725,800,000 cubic feet.

GEOLOGY AND SOILS

The central portion of this basin, representing a much eroded plateau with stream drainage well established, lies within the Appalachian Plateau province. It is underlain by rock formations of the Pennsylvania system, the most prevalent summit and ridge formation being the Pottsville sandstone. The soil types are chiefly residual silt loams and sandy loams of the DeKalb series. They may sometimes be quite sandy and stony. Rock outcrops are common. The upland and ridge soil types are shallow with excessive drainage. They wash severely by sheet erosion when exposed or cultivated on slopes. The valley soil types are deeper, and under normal treatment do not suffer greatly from excessive erosion.

Within this basin ice is important in the carving of banks and subsequent bank caving along the main streams. Ice centers into all river management and engineering problems.

A considerable part of the plateau area is underlain with coal, oil, and gas of economic importance. These resources have determined the utilization of extensive land areas, and in such cases forest cover and soil quality have not been considered of major importance.

A belt along the northern border, including that portion of New York State within the basin, is underlain with limestone and shale of the Portage and Chemung groups of the Devonian period, and with sandstone and limestone of the Waverly groups of the Lower Mississippian. The northwestern portion, about 22 per cent of the basin, has been glaciated. The topography had achieved a considerable roughness before glaciation and the glaciation has resulted in the smoothing of the higher hills and the filling of the valleys with deep deposit. The drift mantle may be several hundred feet in depth. The soils have been derived from the weathering of the upper layer of this drift. They are classified as glacial shale and sandstone soils. (Volusia series.) The commonest soil type is a silt loam that tends to develop faulty underdrainage under cultivation, through the development of a type of hardpan. In forested areas this faulty structure does not occur. Excessive erosion occurs only under conditions of careless cultivation on slopes.

The southeastern portion, about 10 per cent of the total basin, has been sharply folded to form a portion of the Allegheny Mountains. Here several formations have been exposed. The common rock formations are limestone, sandstone, and shale of the Devonian period and the immediately overlying Lower Mississippian. The soils are loams and silt loams, which because of the steep topography are subject to considerable sheet erosion in the higher slopes and to occasional gullying in the lower areas.

CLIMATE

The mean annual precipitation over the Allegheny watershed amounts to 42.4 inches. This is distributed through the seasons as follows:

Range in mean annual precipitation over the watershed is from 36.15 inches at Pittsburgh to 51.2 at Cherry Creek, N. Y., with a maximum for any one year of 65.66 at Corry, Pa., and a minimum of 22.8 at Oil City, Pa. The range in distribution of rainfall from April to September, inclusive, is from 47.2 per cent to 60.2 per cent of the total for the year.

Annual snowfall is greatest in the northern and northwestern portion of the drainage. The least annual snowfall, 34.9 inches, is at the mouth of the river at Pittsburgh. The greatest recorded is at Jamestown, N. Y., 100.2 inches. The heaviest precipitation for a 24-hour period occurred at Saegertown, Pa., where 5.66 inches was recorded.

The mean annual temperatures for the several stations range from 44.7° to 52.6° F., with a mean maximum temperature of 61.6° F. and a mean minimum temperature of 33.8°.

Records from 15 stations widely scattered over the drainage area present the following range in extremes of temperatures for a given year: Minimum, minus 8 degrees to minus 35 degrees; maximum, 94 degrees to 103. There seems to be little or no relationship between elevation and temperature nor is a direct relationship between elevation and precipitation apparent from the records.

Even though the precipitation is well distributed by months, the winter precipitation is concentrated in the form of snow, which may melt in the spring and run off in a few days. The wide annual variation in precipitation and temperature over a period of years makes necessary an analysis by individual years properly to correlate weather and floods.

The climatic conditions preceding the greatest floods in the Allegheny may be summarized as follows:

Low temperatures in November and December before the ground was covered with snow to any depth, followed by heavy snowfall in December, January, and February. Sudden thaws in March, accompanied by 2 to 5 inches of rainfall over large portions of the basin. These conditions result in rapid run-off of rain and snow water with little or none going into the frozen ground.

The seriousness of the flood in the main stream is always tempered by the degree to which the tributaries are in flood and the extent to which the crests of the various tributary floods meet in the main

stream.

HISTORICAL DEVELOPMENT

Settlement in the Allegheny Valley began prior to the Revolutionary War. Agriculture had reached a high state of development as early as 1846. By this time almost the entire lower basin had been cleared for farms. About 1800 coal mining began in the lower valley and has increased steadily since that time. Exploitation of

oil and gas began after the Civil War and the industry has now passed its peak. Lumbering did not begin on a large scale until after the Civil War. However, it developed rapidly and had passed its peak in 1885.

Land utilization is partially indicated by the following table of percentages, computed from the Census of Agriculture for 1925:

The last item includes large areas of land in forest or at least not developed. It also includes cities, villages, and road and railroad rights of way, so that accurate separation of these items is net possible without securing more data at the source.

Ownership of land in the Allegheny drainage area in 1925 was as follows:

Cultivated land occupies 25.9 per cent of the drainage area. The principal farm crops are corn, wheat, oats, hay. The grape crop is important in the northwestern portion of the basin. Dairying is carried on more or less extensively throughout the area. Orchard fruits are produced on 90 per cent of the farms. Beef cattle are not extensively raised in this section. Potatoes are grown on 70 per cent of the farms.

Cultivation devised to prevent erosion is carried on only spasmodically throughout the drainage area. Methods resorted to are contour plowing and the leaving of strips in sod between plowed strips. On a large percentage of the farms the steepest slopes are left in pasture or wood lots.

Erosion of cultivated lands in the Allegheny drainage is mainly sheet erosion. Extensive gullying is not usual. However, deposits in streams and the amount of silt carried after heavy rains indicate that severe erosion takes place.

Pasture land as it exists in the Allegheny drainage area ranges from grass land to land that is almost entirely in forest. Efforts to keep tree growth out of pastures have not been vigorous, so that many pastures revert to woodland. The condition of the grass pasture is good. Overgrazing is not extensive, the condition of most pastures indicating that their carrying capacity is not being fully utilized. The practice of burning to keep down forest growth in grazing lands is not common.

Brush lands occupy about 7 per cent of the drainage area. These are chiefly the result of repeated fires on lands formerly in forest.

Three-fourths of the brush area is in inferior hardwood species but can be expected eventually to become productive forest land. The other one-fourth is occupied by a sparse growth of grass or weeds, or by bare rock ledge, the result of fires and damage by coke-oven gases. Part of it can be replanted successfully, but much of it will remain barren for an indefinite period because of the erosion. Where fires are not too frequent a brush cover develops which tends to prevent erosion.

CONDITION OF FOREST

Originally, there were large areas of northern white pine and hemlock in the Allegheny drainage. These have practically disappeared. The prevailing forest cover is a varying mixture of hardwoods, all classed as "upland hardwoods." A few small patches of old-growth white pine and hemlock remain in the northern part of the drainage but these are so small compared to the total area that they will not be considered separately. The virgin white pine and hemlock are replaced after cutting by natural growth of beach, maple, yellow birch, and black cherry.

The upland hardwood type occupies 5,132 square miles or 44 per cent of the entire area. In the northern part of the drainage basin this type is made up of beech, yellow birch, and several maples, with scattering of other hardwoods, beside scattered white pine and hemlock. It occurs at elevations above 2,000 feet. Elsewhere in the drainage the principal species in this type are red, white, black, and chestnut oaks, sugar maple, and red maple. Other species, such as ash, hickory, basswood, and yellow poplar, also occur in the mixture. In fact, almost all hardwoods common to the Middle Atlantic States can be found in the Allegheny drainage. Chestnut formerly occurred all through the drainage, sometimes in almost pure stands, but has now been largely killed by the chestnut blight and has been replaced chiefly by various species of oaks.

The condition of the forest after logging depends upon the severity of the cutting and upon the nature of the products removed. Usually everything that is merchantable is cut. In the forest regions near the coal mines practically a clear cutting results with a high degree of utilization. But in the northern part of the drainage, where the destructive distillation plants are operating, there is a higher degree of utilization. Cutting is more conservative in the holdings of the two largest operators in the Allegheny Valley.

If the area is not burned, a new forest cover will quickly establish itself after cutting. Subordinate vegetation is abundant, both grass and brush becoming more dense after logging. Slash is left where it falls, no attempt being made to lop or pile, though lopping of tops is being urged by the State forestry department. Most of the timber now being cut is hardwood, which is reproducing both by sprouts and by seedlings already established at the time of the cutting. This reproduction, with the trees left, assures enough ground cover to prevent erosion.

Damage by fire is apparent all through the watershed, varying with the type of forest and the season of the year during which the fires have occurred. In old hardwood stands, all ground cover and advance reproduction is killed and the older timber is usually firescarred in the butts. On cut-over lands fires burn with much greater