Question: Clarence Hungerforf Webb studied which Caddoan culture sire 15 miles from the Mississippi River?

Context paragraphs:
Belcher Mound Site: The Belcher Mound Site (16CD13) is an archaeological site in Caddo Parish, Louisiana.  It is located in the Red River Valley 20 miles north of Shreveport and about one-half mile east of the town of Belcher, Louisiana.  It was excavated by Clarence H. Webb from 1959 to 1969.  The site gives its name to a local phase of the Caddoan Mississippian culture, the Belcher Phase, which radiocarbon dates suggest lasted from 1400 to 1600 CE.

Mississippi River Watershed Conservation Programs: Conservation programs for the Mississippi River watershed have been designed to protect and preserve it by implementing practices that decrease the harmful effects of development on habitats and to overlook monitoring that helps future planning and management.  A main focus is nutrient pollution from agricultural runoff of the nation's soybean, corn and food animal production, and problems relating to sediment and toxins.  Conservation programs work with local farmers and producers to decrease excess nutrients because they cause major water quality problems along with hypoxia and loss of habitat.  Organizations such as the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force and USDA programs such as the Upper Mississippi River Forestry Partnership and the Mississippi River Basin Healthy Watersheds Initiative contribute to conserving what is left of the Mississippi River watershed.

Bayou Sel: Bayou Sel is a prehistoric and historic archaeological site in Clark County, Arkansas, near Arkadelphia.  The site has archaeological remains, including finds relating to the Caddoan culture, as well as prehistoric and historic evidence linking the site to the Quapaw.  The Quapaw and early French settlers in the area are known to have manufactured salt in at the site, and it was here that a salt-processing operation was run in the first half of the 19th century.

Innisville, Ontario: Innisville, Ontario is a small village set in between the towns of Perth and Carleton Place, Ontario on what is known as Highway 7 or the Trans-Canada Highway.  It lies about 15 miles from Perth, on the Mississippi River.  The village is separated by the Mississippi River (via Mississippi Lake) into Innisville South and Innisville North.

Ross Site: The Ross Site designated 3CL401, is a prehistoric archaeological site in Clark County, Arkansas, near the small town of Whelen Springs.  The site includes two Native American mounds from the Caddoan culture, which have been dated to AD 1400-1600.  It is one of a small number of Caddoan sites in southwestern Arkansas.  The site was relatively unscathed until the mid-1980s, having never been plowed over, thus leaving intact potential ground-level features other than the mounds.

Mississippi River Bridge (La Crosse, Wisconsin): The Mississippi River Bridge is a combination of two individual bridges which are also known as the Cass Street bridge and the Cameron Avenue bridge, as well as the Big Blue Bridges.  They connect downtown La Crosse, Wisconsin to Barron Island, crossing the east channel of the Mississippi River.  Another bridge, the La Crosse West Channel Bridge connects Barron Island to La Crescent, Minnesota.  The Mississippi River Bridge carries U.S. Routes 14 and 61 with WI 16 and MN 16.  There is another bridge about four miles upstream in the area, the I-90 Mississippi River Bridge that connects North La Crosse, French Island, and Dresbach, Minnesota.

Poverty Point: Poverty Point State Historic Site (French: "Pointe de Pauvreté" ) (16 WC 5) is a prehistoric earthworks of the Poverty Point culture, now a U.S. National Monument and World Heritage Site located in the Southern United States.  It is 15.5 mi from the current Mississippi River, and situated on the edge of Maçon Ridge, near the village of Epps in West Carroll Parish in northeastern Louisiana.

Clarence Hungerford Webb: Clarence H. Webb (25 August 1902 - 18 January 1999) was an American medical doctor and archaeologist who conducted extensive research on prehistoric sites in the southeastern United States.  A pediatrician by profession, he became interested in archaeology on a camping trip with his sons where he found some small, triangular points.  A distinguished physician, his archaeological research included the study of Caddoan culture, and at a number of major sites such as Poverty Point, John Pearce , Gahagan, and Belcher Mound.

Davis Bend, Mississippi: Davis Bend, Mississippi (now known as Davis Island), was a peninsula named after planter Joseph Emory Davis, who owned most of the property.  There he established the 5,000-acre Hurricane Plantation as a model slave community.  Davis Bend was about 15 miles south of Vicksburg, Mississippi, and was surrounded by the Mississippi River on three sides.  He gave his much younger brother Jefferson Davis the adjoining Brierfield Plantation.

Yancopin Bridge: The Yancopin Bridge is an abandoned railroad moveable bridge spanning the Arkansas River, and the last bridge across the Arkansas River before it flows into the Mississippi River 15 miles to the southeast.  It is distinctive not only for its size and remoteness, but also for having not one but two movable spans, one having replaced the other due to river avulsion.

Supporting paragraphs: Clarence Hungerford Webb Poverty Point
Answer: Poverty Point