From the Daily News in Tanzania comes this update about the water hyacinth invasion in Lake Victoria. The Lake Victoria Environment Management Project, which entered its second phase in 2006, now says populations of water hyacinth (Eichhornia crassipes) have doubled over the past year, and the plants now cover over 700 hectares (1700 acres) of the lake's surface. Back when the LVEMP was first starting up, they had reduced populations by 80%. There is quite a lot of diversity in the potential vectors behind this resurgence: agricultural activities that release silt and fertilizer into the lake, untreated sewage that is dumped in the lake, failure of biocontrols due to excessive water movement in adjacent rivers, and car washes along the shore of the lake that lead to oil and polluted water entering the lake.
You can actually observe for yourself the extent of the water hyacinth invasion in Lake Victoria in this feature from NASA's Earth Observatory. NASA scientists used satellite imagery to estimate the surface area of the lake that was covered by water hyacinth. The photos show that the water hyacinth completely took over the lake's Kisumu Bay between Dec. 2005 and Dec. 2006.