Several ecosystems are present in the Everglades, and boundaries between them are subtle or absent. The primary feature of the Everglades is the sawgrass marsh. The iconic water and sawgrass combination in the shallow river 100 miles (160 km) long and 60 miles (97 km) wide that spans from Lake Okeechobee to Florida Bay is often referred to as the "true Everglades" or just "the Glades".[45][46] Prior to the first drainage attempts in 1905, the sheetflow occupied nearly a third of the lower Florida peninsula.[36] Sawgrass thrives in the slowly moving water, but may die in unusually deep floods if oxygen is unable to reach its roots. It is particularly vulnerable immediately after a fire.[47] The hydroperiod for the marsh is at least nine months, and can last longer.[48] Where sawgrass grows densely, few animals or other plants live, although alligators choose these locations for nesting. Where there is more room, periphyton grows.[49] Periphyton supports larval insects and amphibians, which in turn are consumed as food by birds, fish, and reptiles. It also absorbs calcium from water, which adds to the calcitic composition of the marl.[50]