Most studies of the interaction of groundwater and surface water in mountainous terrane have focused on streamflow generation in small headwaters areas and on the hyporheic zone contiguous to these high-gradient streams. Studies of streamflow generation are closely related to hillslope hydrology and watershed modeling. Studies of these systems have a long and extensive literature and are covered in other papers in this U.S. National Report; therefore, streamflow generation is not discussed here. Studies of the hyporheic zone, however, have increased markedly in recent years, and they are discussed below. However, to first present a larger view, a broad overview of the interaction of groundwater and surface water in mountainous terrane was recently presented by Silar [1990]. In addition, two studies evaluated relatively large mountainous groundwater-surface water systems. Parriaux and Nicoud [1990] discussed the hydrogeology of glacial deposits commonly found in glaciated valleys, and the effects of groundwater discharge from various geologic deposits on streamflow. Seiler [1990] studied groundwater movement in overdeepened segments of valleys in the Bavarian Alps. The segments are divided by bedrock highs beneath the valley, and he found that deep, relatively isolated, groundwater flow systems are present in the thick alluvial deposits within each segment.
The hyporheic zone, which is the subsurface zone of porous media directly underlying and adjacent to the stream, is characterized by a strong downstream component of flow, where the interchange of water between the stream and its subsurface is common. Because of the dominance of downstream flow, studies of processes in the hyporheic zone have been mostly of small, high-gradient mountain streams where some of the stream water follows buried channel deposits for a distance and reemerges into the stream downgradient [ Bencala et al., 1984]. Modeling and the use of tracers have been effective investigative tools in developing an understanding of water movement in small mountain streams [ Bencala et al., 1993]. Water in the hyporheic zone of high gradient streams would not be considered groundwater by some hydrologists but rather surface water that flows through the subsurface for short distances as `substreams' [ Harvey and Bencala, 1993]. Most studies of the hyporheic zone do not consider inflow from the valley side because it is generally considered to be minimal relative to the downvalley flow. However, recent work by Bencala et al. [1993] has extended the concept to a broader landscape perspective.
The source of water in the hyporheic zone is somewhat immaterial because research has shown that significant chemical and biological processes occur that affect both surface water and groundwater chemistry. For example, Triska et al. [1990] indicated that variation in local exchange of flows between the stream and hyporheic zone produced temporally shifting concentration gradients of dissolved oxygen, nitrate, and ammonium in the subsurface, and that biological activity played a significant role in the chemical transformations. Carbon cycling in the hyporheic zone has been the topic of much attention. For example, Dahm et al. [1991] found that anaerobic carbon cycling was an important process in the hyporheic zone of several mountain streams in New Mexico. Ford and Naiman [1989] found that biologically related oxidative processes in the hyporheic zone of a headwaters stream in Ontario were responsible for removing dissolved organic carbon from inflowing groundwater.
Chemical transformations, particularly related to nitrogen chemistry, take place to depths of several decimeters in streambed sediments, even in small low-gradient streams that have a large groundwater inflow component. In a study of a small stream in Michigan, Hendricks and White [1991] found that dissolved organic carbon decreased with depth but not with distance downstream within the hyporheic zone. Furthermore, they found that gradients for temperature, chloride, silica, nitrate and phosphate over the 10-mile reach of surface water were similar to those observed within a single hyporheic zone. These are just a few examples of papers in an emerging field of `ground-water ecology,' which is concerned with the biogeochemistry of the hyporheic zone and the effect on both groundwater and surface water quality [ Husman, 1976; USEPA, 1992; Hakenkamp, C.C. et al., 1993].
Many studies of the interaction of lakes and wetlands in mountain terrane have been prompted by the need to evaluate the effects of acid deposition on these surface water bodies. For example, Peters and Murdock [1985] and Schafran and Driscoll [1993] indicated that lakes in the Adirondack Mountains, New York, that have groundwater input have a more neutral ph than lakes that have little groundwater input. Schafran and Ika [1991] indicated that nearshore sediments have a major effect on seasonal variations in aluminum, lead, and base cations. In a study of Emerald Lake in the Sierra Nevada Mountains, California, Williams et al. [1990] found that the overall importance of groundwater to the chemistry of this alpine lake depended on the timing of inputs from snowmelt and groundwater rather than the total volume of these water sources.
Even though studies of the hyporheic zone indicate that water moves freely between the river and the substream in high-gradient mountain streams, the stream may not always be the primary source of water to riparian wetlands. In a study of riparian wetlands in a subalpine valley in Colorado, Ruddy and Williams [1991] found that precipitation, snowmelt, runoff from the mountainside, and groundwater were the major sources of water to the riparian wetlands. Although flow occurred in `substreams,' the effect on the wetlands was generally restricted to the paleochannels.