The Texas Parks and Wildlife Department has confirmed the presence of giant salvinia in Chaney Branch on Lake Fork. The infestation appears to be confined to this branch and another small cove west of the dam and occupies an estimated 3.25 acres. Judging by the distribution and age of the plants, it is apparent it has been in place for several months. As is the case for other introductions in East Texas, the plants were most likely introduced to the lake by boat trailer.
Lake Fork�s controlling authority, the Sabine River Authority (SRA), immediately closed boat ramps at Chaney Point South and Secret Haven, which are located at the westernmost end of Chaney Branch, in order to eliminate any further risk of spreading this invasive plant. The SRA has also conducted reconnaissance at bridge crossings and conducted shoreline surveys by boat to look for additional infestations.
Crews from the SRA and TPWD have physically removed salvinia plants and placed them in garbage bags for disposal. Additionally, crews have placed approximately 1,100 feet of floating boom across the creek, containing the infestation within the 90-acre cove.
SRA staff has also installed a floating sign and lights to alert boaters to the presence of the floating boom.
TPWD staff will be conducting a chemical treatment using glyphosate. All efforts will be made to protect beneficial aquatic plants while focusing treatment on killing the invasive giant salvinia. TPWD will continue to conduct additional surveys to check for the spread of the plant to other areas of the lake, and additional physical removals will be done if necessary.
Efforts to increase public awareness will be increased by installing warning signs at major boat ramps. Boaters and property owners are urged to clean, drain, and dry their boats and trailers in between trips and to report additional infestations outside of the Chaney Branch area by calling (903) 593-5077, (903) 570-5745, or by emailing kevin.storey@tpwd.texas.gov.
Giant salvinia is a floating fern native to Brazil. It reproduces by budding and spreads easily by the movement of wind and water currents. Giant salvinia grows rapidly and can double in coverage in a week. Uncontrolled, giant salvinia can impede navigation, block sunlight from reaching the water and hamper fishing.
This incident is a painful reminder of the importance of cleaning all plant material from your boat, trailer, and towing vehicle and draining your boat before leaving a lake. Transportation of aquatic invasive species is illegal in Texas.
For more information on giant salvinia and other invasive species, visit http://www.texasinvasives.org/.

Looks like Parrotfeather. http://aquaplant.tamu.edu/plant-identification/alphabetical-index/parrotfeather/

How does Fork get so much vegetation and not a single lake around San Antonio can even grow hydrilla?