Rivers and Streams, Characteristics - Hydro-Environmental Analysis: Freshwater Environments - page 31

Lag time

Rising
limb

Recession
limb

0 1 2

Time (days)

3 4

Time
of rise

FIGURE 2.18 Flow hydrograph. (From FISRWG, Stream Corridor Restoration: Principles, Processes, and
Practices , Federal Interagency Stream Restoration Working Group, 1998.)

the area below (and beside) the streambed where water percolates through spaces between the sedi-
ments, rocks, and cobbles and other permeable soils (saturated interstitial space, Figures 2.19 and
2.20). Hyporheic zones are often referred to as ecotones or bridges between two different types of
habitats or environments, such as surface water and groundwater. Boulton et al. (1998) deined the
hyporheic zone as a “spatially luctuating ecotone between the surface stream and the deep ground-
water where important ecological processes and their requirements and products are inluenced at
a number of scales by water movement, permeability, substrate particle size, resident biota, and the
physiochemical features of the overlying stream and adjacent aquifers.”
The hyporheic zone is an important component of river hydrology and hydraulics. Depending on
the hydraulic gradient, lows may be into or out of this zone (Figure 2.19), adding to or removing
lows from the stream or to groundwater, and a hyporheic low may include much or all of the total
river low. The recognized functions of the hyporheic zone (NRCS 2007) include:

• Regulation of the stream temperature by groundwater upwelling

• Water retention and storage, which can reduce peak lows during loods and sustain base

lows during dry periods

• Habitat creation, especially for aquatic invertebrates such as crustaceans, and vertebrates

such as larval ishes

• Buffering and iltering nutrients from streamlows and groundwater

• Aquifer recharge

• Nutrient enrichment

2.2.2.2 Floodplain
The loodplain is the area above the streambank, the extent of which may vary. Some streams
and rivers low through valleys, where it is relatively easy to differentiate between the channel,
the loodplain, and upland areas. Other rivers low through deltas, fans, or broad plains (Wetzel
20 01).