Stream corridors
Stream corridor vegetation (also often called 'gallery forest') provides an important refuge for animal and plant species that became vulnerable to extinction at times of abrupt climate changes occurring during the past glacial periods. Due to the permanent presence of water, stream corridors vegetation is highly dynamic and has a great capacity to regenerate. In our basin, stream corridors host at once typical forest species (Red cider, African Olive, ...) as well as plants that grow in presence of permanent high moisture (bamboo, a large number of vines ...). Along stream corridors, trees tend to grow faster than elsewhere and to become taller; for this reason, stream corridors are heavily targeted by loggers and charcoal producers.
According to the Kenyan Water Act of 2002, riparian zones should be preserved from human disturbance on the premise that riparian zones are government property. Despite this, lack of enforcement and diffuse ignorance of the legal system constitute a weak barrier to riparian zone degradation.
Along the Gilgil river, three main riparian vegetation types can be distinguished. Right at the top, the stream crosses remnants of the once extensive primary forest including red cider (Juniperus procera) and, in places, bamboo thickets. Common riparian species include Dombeya goetzeni (ideal for rearing honey bees) and Hypericum revolutum, a well-known stimulant with healing properties.
Due to common exploitation for charcoal burning and for cattle watering, a large portion of the Gilgil stream corridor is heavily impacted; the best preserved sites correspond to waterfalls and raids marking geological discontinuities (fault lines) that are of difficult access to bith humans and to cattle and that do not allow easy timber extraction.
At midpoint in the catchment, shrubs of riparian zone include Sida, Rhus, Grewia and Pavonia. In the lowlands, the riparian zone becomes severely degraded, colonised by few resistance trees ans shrubs, such as acacias, figs, and Devil's horsewhip Achyranthes aspera. Vegetation density increases towards the riverbank but exhibits several gaps created by cattle and wildlife. These animal paths indicate that the riparian zone serves as drinking points. Invasive helophytes such as Sodom apple (Solanum sp.), Aspilia and Psidia are common.
Pavonia urens (Malvaceae), is a soft hairy shrub growing along the sides of water courses in drier areas; widespread at 1220-2740 m a.s.l., used for making strong flexible ropes.
Invasive alien species susceptible of expanding into the basin's stream corridors include three species of Prosopis, originally introduced from Central and South America (where it is called mesquite) for the restoration of degraded drylands, which have taken over extensive portions of lowland riparian corridors in the northern Rift valley in the course of the last 25 years. This new World legume is not yet widespread within our basin, but there are fears that its area of distribution could increase further.
