Billie,
Forts Henry and, to the east, Ft. Donelson sat astride the Tennessee River, Fort Donelson also was near the Cumberland River, a quick route to Nashville. Taking the forts was integral to moving Union forces into the center of the Confederacy, Mississippi, Alabama and of course Tennessee. Grant's gun boats entered the Tennessee and Cumberland from the Ohio river and his army trailing behind because of high water due to heavy rains crossed into Western Kentucky near Cairo, Ill. Sailing south on the Tennessee the gunboats encountered light resistance from the Fort Henry artillery. the Confederates had built the fort on a flood plain and the majority of the fort lay under water. Further, the defenders accidentally spiked their own guns due to inexperience, and the fort surrendered to the gun boats in about two hours. Upon the Union army's arrival they quickly left the fort and followed the same trails the Confederates had taken 12 miles to Ft. Donelson where the Confederates hoped for the protection of the fort's heavy guns. Due to the marshy high water there was slow going for the Union and it took 6 days to cross the 12 miles. On arriving at Fort Donelson Grant found himself facing an army of 12,000 Confederates. Several excellent books can be found at your library for further background. Knowing you have family involved you may find them of particular interest, and a good place to start.
