The Irbas Project

What are intermittent rivers?

• Intermittent rivers (IRs) are rivers for which surface water ceases to flow at some point in space and time
• IRs comprise a large proportion of the global river network
• IRs are characterized by dynamic alternations between terrestrial and aquatic habitats
• IR habitats support aquatic, semi-aquatic, and terrestrial biota
• IRs provide essential ecosystem services to society, including flood control and irrigation
• The abundance and distribution of IRs, and their natural intermittent flow regimes, are being altered by climate change, water abstraction and inter-basin transfers
• Despite their values and ongoing alterations, IRs are chronically under-studied and protective management is inadequate

The importance of intermittent rivers

A large proportion of the global river network is composed of rivers that periodically cease to flow, or intermittent rivers (IRs). IRs are not restricted to arid regions; they occur in most terrestrial biomes. Global estimates of the lengths and discharge of IRs are lacking, but a selection of national and regional estimates indicates their prevalence. There are at least 3,200,000 km of IRs (60% of total river length) in the conterminous United States and at least 70,000 km of IRs (25-35% of total river length) in France. While these estimates are limited in spatial extent, they suggest that the IRs are extremely abundant.

In the next century, the number and length of IRs will increase in regions that experience drying trends due to climate and land-cover change, and increasing water abstraction for irrigation and other economic uses. Regional and global trends in intermittence have not been analyzed, and predictive models of future changes in intermittence are at early developmental stages. However, negative trends in flow have been detected in many regions and linked climate change-runoff models predict future decreases in runoff in some mid-latitude regions due to decreased rainfall and increased evapotranspiration. If these climate-driven changes eventuate, increases in the occurrence and frequency of intermittence are likely to follow. In addition to climate-driven intermittence, many perennial rivers have become artificially intermittent due to impoundment and excessive water abstraction; these shifts have severe ecological and economic consequences. Among the best-known examples of artificial intermittence are the Nile, Indus, Yellow, Armu Darya, Syr Darya, Mekong, Rio Grande, and Colorado Rivers.

Historically, IRs have been overlooked by aquatic and terrestrial ecologists; these rivers were perceived to be outside of the scope of both terrestrial and aquatic sciences. It was also assumed for many years that IRs have low biodiversity value, and are inhabited by depauperate communities of drought-resistant species. Ecological studies of IRs have increased sharply in the last decade partly in response to increases in the severity of intermittence in regions with high water demand or climatic drying. There are now datasets and corresponding metadata available from IR studies in multiple nations. These data provide evidence that IRs are inhabited by unique and diverse lotic, lentic and terrestrial communities. However, most IR studies are limited in spatial scope, and they vary in habitats, taxonomic resolution, and explanatory and response variables. As a consequence, current knowledge of IR biodiversity and its controls is sparse and fragmented. The cross-site comparisons and meta-analyses needed to upscale small-scale studies and make general inferences are lacking.

The recent, rapid increase in IR research has led to a widespread acknowledgment that these rivers require rigorous management to protect biological values. However, current management practices and protective policies and legislation for IRs are often inadequate, or nonexistent. As a result, many IRs and the communities that inhabit them are degraded by human activities during both flowing and non-flowing phases. The most common threats to IR communities are alterations in natural flow regimes due to surface and groundwater abstraction, impoundment, and artificial flow augmentation. Unfortunately, minimal effort has been made to protect or restore naturally intermittent flow regimes. Effective management of intermittent flow regimes requires knowledge of the relationships between flow regime components (e.g., dry periods, floods, base flow) and ecological responses. These relationships are rarely identified and this knowledge gap has impeded IR management.

Objectives

The IRBAS project will compile and analyze biological, hydrological and environmental (e.g., land-cover, geology, climate) data from intermittent rivers (IRs) across the world. The datasets will be used to (1) estimate the abundance and distribution of IRs at regional and continental scales; (2) analyze temporal trends in flow intermittence; (3) identify and quantify relationships between flow intermittence, habitat dynamics, and structural and functional biodiversity; and (4) predict biodiversity responses to future changes in flow regimes.

The overarching goals of IRBAS are to improve our understanding of biodiversity patterns in IRs, raise awareness in the scientific and management communities and the public of the importance of these rivers, and provide guidelines for policy-makers and resource managers for effective water and habitat management, restoration and preservation.