The basic-physical ways of preventing silt are twofold and easily defined: first, the maintenance of proper land cover—vegetation or humus or mulch which blankets and anchors the soil particles and prevents falling or flowing water from dislodging them—and second, structural approaches that control the flow of water and can also serve to trap eroded material. These latter can be anything from good contour plowing practices to a major reservoir with a certain silt capacity built into it.
Such techniques are the basis of existing programs of the Soil Conservation Service and the Forest Service that have proved their effectiveness over many years of rural application. Watershed planning with small reservoirs, check dams, and terraces backed up by good land treatment and use, soil surveys, wise forestry practices, and such things are stimulated and bolstered in these programs by technical and financial assistance given to private landowners, States, and local organizations. They have already had important local effects in the Potomac States as throughout the country, but for maximum value in relieving sedimentation they are going to need much wider and more intensive application.
In modified form, they can be effective against newer and more concentrated sources of silt, while sometimes accomplishing other purposes as well. As we noted in discussing metropolitan pollution, urban land undergoing development can enormously benefit from good watershed planning. Preservation of critically erosive and flood-prone land in grass and forest, insistence on prompt re-vegetation of bared land and the use of such things as sediment detention basins by developers, the construction of small headwater reservoirs when they are needed to trap silt and reduce flooding—all these elements of watershed planning are effective not only against silt but against standard urban and suburban ugliness. The translation of rural techniques to city use cannot be literal, for both urban hydrology and urban land use are distinctive, and a good deal remains to be learned about making the techniques work better there. But their basic principles are obviously a main hope.
Other modifications of them, if put into wide practice, can cut down on the heavy production of silt by strip mines in the upper Basin; these involve both the reclamation of abandoned mines and the use of more care in scraping new ones. And application of the same principles—protective cover and detention of runoff—to new highway and road construction, as well as to the reclamation of banks and shoulders on old secondary roads, has to be achieved.
The silt already in the upper estuary, and likely to continue to be deposited there even after the best available controls may have been put into operation above, will need radical treatment. The tens of millions of tons already choking the metropolitan river, the stockpile of centuries, will have to be dredged out if the river is going to be as pleasant and useful at the capital as it ought to be, and so will the yearly additions that can inevitably be expected. This can be done if the money is available, though a considerable unsolved problem, under research at present, is where to put the silt after it has been taken out of the river, for appropriate fill sites are growing scarce.
Turbidity in the sluggish upper estuary will continue to be a problem too, for the fine particles of silt that cause it are the least affected by standard land treatment and sediment control measures. Polyelectrolytes—chemicals which when applied in quite small amounts can coagulate such suspended silt and settle it out—offer some promise as tools against turbidity and are being tried out experimentally above one of the reservoirs on upper Rock Creek, with good results thus far. Very possibly they may prove to be useful for clearing up the estuary after it has been roiled by storm runoff, and for achieving some control of murky waters around sand and gravel dredging operations. However, ironically, it has also been pointed out that until the excess of nutrients in the upper estuary is eliminated, such clearing of the water could very possibly cause a great increase in the already disastrous algae blooms, by allowing sunlight to penetrate to greater depths and foster more production of this undelightful greenery. Cleanup of pollution as complex as that evolved in the 20th century has to be across the board.
Barring a general philosophical revolution on the part of the American people, the problem of junk and debris in our waters is likely to continue and even to increase as people and their consumption of the products of the economy maintain their geometric growth. Clean rivers in themselves might deter a good many people from cluttering them thus, and so might public education, stiff fines, and the provision of better municipal pickup and dumping facilities. But mainly getting rid of such detritus is probably going to be a matter of fairly continuous gathering and disposal. On navigable waters like those of the upper Potomac estuary, ingenious collection craft under the command of Army Engineers are in prospect; elsewhere the job is likely to be more old-fashioned and laborious.
For certain remaining pollution problems, no definite full technological answers exist at present and the main hope must be to alleviate them as much as possible while pressing a search for long-run answers. Some are relatively restricted in their effects in the Potomac Basin so far, though they have some drastic local effects and some long-run implications. Certain industrial wastes not amenable to any presently known form of treatment, such as tannery discharges at Petersburg, West Virginia, and Williamsport, Maryland, are one example. So are the noxious exudations of raw sewage and garbage from ships and pleasure craft. Marinas themselves and the boats docked there can and must be connected to waste collection systems. Laws can and should prohibit discharges from watercraft in harbors and rivers. But until better means of on-board waste treatment or retention than exist at present are evolved and made mandatory, the multitudes of boats with standard toilet facilities are going to keep on causing trouble.