New York, Doe. 7. New Orleans letters state the force nterfering with the navigation of the Mississippi, below the niouth of lied River, is estimatcd at 3,000, under General Dick Taylor, and a 6ort of fiyng corps appaaring frora time to time at different points of the river, preying on its commerce. The merebants of New Orleans and trariers generally are surinuRly alarmed at Iho audacity, frequency and snecess of these attacks, and have appiied to the rnillitary autborlios to have all river boats armed and manned by n lew artillerists. Major General Washburn, with his command, is moving in tho direction of Matagorda, with every prospect of capturing the place. Onr forces will then have a footiog on the main land, with a fine harbor to anchor ourvessels in, and a good base of operation?.