**4. Conclusion**

density is highest at Damani dam (4.22m<sup>2</sup>

**Figure 4.** Tree densities downstream and upstream of the dams.

and lowest on the Vondo dam (10.11m<sup>2</sup>

) and upstream (5.86 m<sup>2</sup>

) than downstream of the dam (6.57m<sup>2</sup>

dation of riparian zones which also leads to loss of ephemeral habitats.

(7.09m<sup>2</sup>

80 Environmental Risks

(4.59 m<sup>2</sup>

(5.88 m<sup>2</sup>

) on the Mbwedi River and lowest at Mambedi dam

). Tree density upstream of Nandoni dam is higher

) tree density. This is a function of the age of the dam: vegeta-

). Thus, the growth in treeless area downstream

)

) on the Mambedi River. Downstream tree density is highest on the Albasini dam and

lowest in all other dams. Downstream tree density is highest on the Albasini dam (5.88 m<sup>2</sup>

of all studied dams occurs as a result of low flows which are available for riparian irrigation. This is caused by the upstream presence of the dams. This, according to Lees et al. [27] and Sutherland et al. [28], occurs because construction of dams leads to fragmentation and degra-

The magnitude of degradation is a function of resilience and fragility of the area which is controlled by soil type, size of the dam, age of dam, size of degraded area during the construction of the dam, species diversity, succession stages, climatic variability, energy pools and routes and soil instability [29]. Albasini dam area has a smallest difference between the downstream

tion, predominantly terrestrial, having had sufficient time to regenerate after the construction of the dam in 1952. Vale et al. [30] have noted that dam impact stabilises a few years after impoundment but becomes intense immediately after dam construction. This explains the smallest difference between downstream and upstream areas. However, Vondo dam area has lower tree density because it is located in a more humid area characterised by high rainfall and sensitive soils. Soils of the area are therefore dystrophic (nutrient-poor). This explains why the area became sensitive to water shortage downstream of the dam. Vegetation that thrived well under humid conditions suffered when river discharge was reduced downstream of the In all the five dams studied, downstream treeless area has increased progressively over the years. In all cases, aerial and orthophotos showed increasing discontinuity in downstream tree cover. Treeless area is bigger downstream than upstream in all cases (except the Mambedi dam, which failed in the year 2000). There is lower species richness downstream than upstream (again, except in the case of the failed Mambedi dam). In all the downstream riparian zones, the predominant vegetation is obligate terrestrial species. Downstream vegetation is less dense than previously and less dense than upstream, although in the case of the oldest dam (Albasini), the difference is small, as the terrestrial vegetation downstream has had more time to recover. These effects are ascribed to the probable presence of the dams causing reduced downstream flows during dry seasons.

From the above summary of the findings of all five dams studied, it can be concluded that any modification of river flow through damming alters flow characteristics and impacts negatively on nutrient cycling of lotic environments. This is accompanied by low downstream species diversity. All dams are characterised by low surface or runoff downstream. Changes in river flow due to damming have led to the replacement of riparian vegetation with terrestrial vegetation or alien plant species in other dams. All downstream sections of the studied dams are characterised by terrestrial vegetation in formerly inundated riparian zones. Total annual rainfall does not guarantee the regeneration of disturbed areas since regeneration depends on many combined factors such as size of dams, size of degraded area during dam construction, natural species diversity, successional stages, climatic variability, energy pools and routes and soil instability. The resilience and fragility of disturbed areas depend on the combination of some of these factors.
