Riperian Plants of New Mexico

What is a riparian area?

Terrell T. "Red" Baker, Extension Riparian Management Specialist

Riparian zones have received increased attention in recent years, particularly in the southwestern United States. Because of the management implications of defining a riparian zone, landowners and resource managers are increasingly calling for a precise description of these unique ecosystems. However, such a precise description is not easily found, largely because the scientific community is still struggling to develop a universally accepted definition. A precise definition will not be provided here, rather, what follows is a general description of riparian areas and a few guidelines for determining whether a particular area should be labeled "riparian".

Riparian is derived from the Latin word "ripa" meaning streambank. Although it was used much earlier in Europe, use of the term in the United States can be traced back to the early 1800's when it was used as a legal term to describe landowners adjacent to a stream or river as "riparians". As conflicts developed between individual mills (i.e., agricultural and industrial) over water use and diversion, legal action and intervention became necessary. From the resolution of those conflicts came the Doctrine of Riparian Rights. The Doctrine of Riparian Rights forms the basis for water law in the eastern portions of the United States, which receive significantly greater annual precipitation than western (usually west of the 100^th meridian) states. The Doctrine of Riparian Rights provides landowners adjacent to a body of water the right to put some of that water to use as long as that use does not interfere with navigation of the waterway or does not reduce the quantity or quality of water for downstream users. However, in the arid western states, water law is based on the concept of Prior Appropriation which states "first in time, first in right".

It is unclear when scientists first adopted the term "riparian" to describe the area adjacent to a stream or lake. However, significant use of the word does not appear in the scientific literature until the 1970's. It is clear that over the last 20-30 years, our understanding of the importance of and ecological processes within riparian areas has improved. Unfortunately, we have still not arrived at a universally accepted, clearcut definition of riparian areas.

Most definitions of riparian areas describe them simply as the transitional zones between aquatic and terrestrial (or upland) environments. Therefore, riparian areas occur as a belt along the banks of rivers, streams, and lakes. Riparian areas occurring along the banks of moving water (i.e., streams or rivers) are often called lotic systems whereas those occurring along the banks of stationary water (i.e., lakes, ponds, or pools) are called lentic systems. As a transitional zone between aquatic and upland environments, riparian systems often exhibit characteristics of both; but they are not as dry as upland environments and they are not quite as wet as aquatic or wetland systems.

As compared to adjacent upland systems, vegetation in riparian areas benefits from greater soil moisture due to its proximity to water. This presence of water in the soil allows certain plants to grow that would not be capable of living in drier upland environments. This results in a vegetation community that is different from upland areas. The vegetation composition differences between upland and riparian areas are particularly obvious in arid states like New Mexico where there are often abrupt shifts in vegetation. In less arid states such as in the eastern United States, the transition between upland and riparian is often much less obvious because upland areas benefit from considerably greater rainfall. In more temperate areas of the country that receive greater quantities of rainfall, the more frequently asked question is, "how wide is the riparian zone?" In the arid southwest, the more frequently asked question is "is this a riparian area?"

Although there is no universally accepted definition of riparian areas, there are some general guidelines that can be used to determine whether a watercourse supports a riparian area. These are based on hydrology of the stream.

There are three types of stream systems in the southwest - perennial, intermittent, and ephemeral. Perennial streams have visible water flowing above the streambed year-round. (Click here for a map depicting perennial streams in New Mexico) There are two types of intermittent streams, spatially and temporally intermittent, and these often occur along the same watercourse.

Spatially intermittent streams have water that appears above the stream bed in certain areas whereas in other stretches, the water remains below the streambed. Temporally intermittent streams do not necessarily have to have water above the surface of the streambed. Water appears above the streambed only after a rainfall event or in response to snowmelt. This occurs because underground water (the water table) is sufficiently close to the soil surface such that it rises above the surface after being recharged by rainfall or snowmelt. Ephemeral streams or watercourses, such as arroyos, flow only in response to precipitation. Intermittent and ephemeral streams are often confused with one another. The critical difference is the connection that intermittent streams have with the groundwater table. Vegetation growing along intermittent streams often has access to the water table or at least a greater quantity of soil moisture due to the proximity of the water table. This creates distinct hydrologic characteristics and vegetation composition differences that distinguish intermittent streams from ephemeral streams. (Click here for a map depicting intermittent and ephemeral streams in New Mexico)

As a rule, only perennial and intermittent streams can support riparian areas that serve the entire suite of riparian ecological functions. Ephemeral streams generally do not possess the hydrologic conditions that allow true riparian vegetation to grow. Although water flows down ephemeral streams periodically, the water table does not occur sufficiently close to the soil surface to allow water loving vegetation to access the greater quantity of water they need to grow. Vegetation growing along ephemeral watercourses may occur in greater densities or grow more vigorously than vegetation in the adjacent uplands, but generally there are no dramatic compositional differences between the two. Even along those ephemeral streams where vegetation composition differs somewhat from the adjacent uplands, that vegetation does not require as much soil moisture as true riparian plants. The critical distinction that needs to be made between ephemeral and intermittent streams relates to ecological function. Due to the unique hydrologic and vegetal characteristics found only along perennial and intermittent streams, only these stream types serve a suitable number of riparian ecological functions to be considered "truly" riparian.

Vegetation composition as an indicator of riparian habitat is important because riparian vegetation serves numerous functions that more upland plant species do not.

True riparian vegetation, whether it is woody shrub species or grasses and sedges, typically has higher root densities than upland vegetation. This helps maintain streambanks, reduce erosion, trap sediments, and allows the stream and riparian system to withstand disturbance related to high water flow events. True riparian vegetation also serves as valuable wildlife habitat. A large percentage of all wildlife species depend on riparian areas (for foraging or nesting or cover) for some portion of their life cycle (Thomas et al. 1979, Johnson et al. 1977). The presence of one riparian species does not necessarily indicate the presence of a "true" riparian area. For example, a cottonwood planted adjacent to a well or stock pond may indicate greater soil moisture in that vicinity, but it will never serve the range of ecological functions typically associated with vegetation in "true" riparian areas.

It becomes important in these cases to objectively assess whether an area, and the vegetation it supports, can truly serve the range of functions unique to riparian systems.