Throughout the humid tropics, increased land disturbance and concomitant road construction increases erosion and sediment delivery to rivers.  Building road networks in developing countries is commonly a priority for international development funding based on anticipated socio-economic benefits. Yet the resulting erosion from roads, which recent studies have shown result in at least ten-fold increases in erosion rates, is not fully accounted for.  While effects of road-derived sediment on aquatic ecosystems have been documented in temperate climates, little has been published on the effects of road-induced sediment on aquatic ecosystems in developing countries of the tropics.  Along the south bank of the Rio San Juan (Nicaragua and Costa Rica), attempts to build a road without engineering or plans resulted in massive failures and erosion in areas where steep slopes impinge upon the river bank.  Pre-existing tributary streams received elevated sediment loads, creating new deposits on pre-existing tributary deltas.  In some reaches with rapidly eroding sites, completely new deltas of freshly deposited sediment were formed, prograding into the river channel.

Riverlab alumni Blanca Rios and Scott Walls joined with Matt Kondolf to study periphyton biomass and macroinvertebrate communities on the deltas of Río San Juan tributaries, comparing north-bank tributaries draining undisturbed rain forest with south-bank tributaries receiving runoff from the partially-built road experiencing rapid erosion. Periphyton biomass, richness and abundance of macroinvertebrates overall, and richness and abundance of Ephemeroptera, Plecoptera and Trichoptera were higher on the north-bank tributary deltas than the south-bank tributary deltas. These findings were consistent with prior studies in temperate climates showing detrimental effects of road-derived fine sediment on aquatic organisms.  A Non-Metric Multidimensional Scaling (NMDS) analysis showed the impacted community on the south-bank deltas was influenced by poorly-sorted substrate with greater proportions of fine sediment and higher water temperatures.  The paper is freely available (open-access) here.

Rios-Touma, B, GM Kondolf, and SP Walls. 2020. Impacts of sediment derived from erosion of partially-constructed road on aquatic organisms in a tropical river: the Río San Juan, Nicaragua and Costa Rica. PLoSONE 15(11):e0242356.

Figure 1. Reach of Rio San Juan from approximately River Km 83.3 to 84.3 (downstream of the outlet of Lake Nicaragua)
Figure 2. Macroinvertebrate richness of south-bank versus north-bank tributary deltas of the Rio San Juan, MarchMay 2014.