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reduction in the possible flood crest of nearly 15 inches. Furthermore, were all the forests of the Mississippi Valley properly protected and managed in accordance with established forestry principles and practices, a further reduction in possible flood crests of 55 inches would be possible. According to the data of Maj. Gen. Edgar Jadwin, this restraining effect would be equivalent to the storage capacity of some 4.6 reservoirs each with a capacity of 10,000,000 acre-feet.

Thus the results even when based on acknowledged incomplete and conservative data, are of such significance that it does not seem possible that the part the forests play in the control of floods can be longer ignored. Certainly it would seem that any plan of the river control that does not include forestry as an auxiliary measure would overlook an important aid in the control of floods at their source.

The recommendations of the department included in the accompanying report indicate the way in which the forests may be made to do their part, and the department will, of course, go as far as it can in its endeavor to improve the situation.

Very sincerely,

The PRESIDENT,

The White House.

W. M. JARDINE, Secretary.

PROTECTION FORESTS OF THE MISSISSIPPI RIVER WATERSHED AND THEIR PART IN FLOOD PREVENTION

By E. A. SHERMAN, A880ciate Forester, Forest Service

FOREWORD

In times past, even before the white man had disturbed the heavy forests of the Mississippi River Basin, floods were known there. With the settlement of the country, forest fires, overcutting, and the abuse of forest and other lands have served to increase the possibility of floods and their severity and the amount and extent of erosion. Forests have a part in the general flood and erosion problem, and it is well that this part should be recognized. It should be emphasized, however, that it is not proposed that forestry should supplant engineering works in flood control, but that forestry should supplement whatever means of artificial control may be adopted by engineers.

A program of sound forestry development that will permit the forests of the Mississippi River Basin to exert their greatest influence on the regulation of water flow should include protection of all forest lands against fire, the reforestation of all denuded lands unsuited for agriculture, the extension of proper forest practices to all forest lands, the public ownership of particularly critical areas, the continuance of existing public forests, and placing the public grazing lands under management. This program should be supplemented by additional research and investigation of ways and means for the better handling of forest lands as a means of controlling erosion and minimizing floods.

INTRODUCTION

The Mississippi River, from time immemorial and until prevented by artificial barriers, was accustomed every few years to inundate about 30,000 square miles of land. This ancient flood plain, which under levee protection had been developed into a very rich territory, was the scene of the flood of 1927 (fig. 1), the greatest physical disaster in American history. Uncontrollable waters inundated about 18,000 square miles of country and dispossessed about 750,000 people of their homes, some temporarily and some permanently. The property loss was appalling. Fortunately, about 12,000 square miles of the richest portions of the ancient flood plain, as well as the city of New Orleans, were saved by protecting levees.

This disaster removed every doubt that the problem of controlling floods on the Mississippi River is a national responsibility, challenging the best efforts of every public agency. The Forest Service responded by promptly undertaking an investigation of the forests and forest lands of the Mississippi watershed to determine their relation to the great flood problem and the measure in which they might be made to contribute to its solution. The purpose of this publication is to present in broad outline the fruits of that study.

The study did not include the special field of the engineer who seeks to control floods by such structures as levees, spillways, and reservoirs, a field already filled by the Mississippi River Commission and the engineers of the War Department. It did include an investigation of land-surface conditions, particularly in forest regions, with a view to determining if better land use might lower flood crests and reduce flood hazards. The activities recommended as a result of this study are proposed as supplementary to engineering works and in no sense as an alternative.

SCOPE OF STUDY

While this is primarily a report on the protective influence of Mississippi Valley forests, the study upon which it is based necessarily covered surface conditions and land use generally. The starting point was the self-evident fact that the condition of the land surface has a direct influence on the amount of water held and retained by the soil, on the time and rapidity of run-off, and on the silt content of streams contributing to floods, as well as on the volume, velocity, and turbidity of the water itself. It follows that any form of land use which affects the condition of the surface has a direct bearing on the run-off from that land.

One of the important uses of land surface is for the production of forest crops. Land so used constitutes about one-fifth of the total land surface of the basin. The forests usually occupy regions of relatively heavy rainfall, and embrace a large percentage of the areas of roughest topography and greatest elevation. These are, generally speaking, the regions from which run-off is most rapid. Such forest lands doubtless play a relatively larger part in the flood problem than mere area would indicate. Determining what areas are now in forests or could advantageously be reforested and estimating their influence on run-off and stream-flow condition come within the field of Forest Service responsibility. This study was limited to that field, excepting as studies of range, pasture, agricultural, and even barren lands were necessary to determine whether or not the extension of forests to lands of some other class was practicable or desirable. The field of this study obviously covered activities and conditions which are factors entering into the flood problem but which do not usually come within the field of the construction engineer's observations and operations.

MISSISSIPPI WATERSHED ALWAYS LARGELY NONFOREST LAND

The Mississippi Basin has always been conspicuous among the great river basins of the world for its large percentage of nonforest

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