Mean Water Column Velocity Research Articles

Near-census ecohydraulics bioverification of Oncorhynchus mykiss spawning microhabitat preferences

ABSTRACTThere is increasing scientific and management interest in Oncorhynchus mykiss habitat use, as this species has become rare and elusive due to anthropogenic impacts on regional populations. Past research defined general physical habitat conditions utilized by spawning O. mykiss, but modern science and management require spatially explicit predictive capability. Meanwhile, ecohydraulics prediction in general lacks objective, transparent bioverification, defined as assessment of the complete performance of a physical habitat model relative to observed biological utilization. This study developed a robust framework for ecohydraulics bioverification and applied it to improve the understanding of O. mykiss spawning. The testbed was 35.2 km of the regulated gravel–cobble lower Yuba River, California. Using two-dimensional hydrodynamic modelling, substrate mapping, and a two-year survey of O. mykiss redds, microhabitat representations were tested for their ability to predict spawner preference and avoidance. O. mykiss redds showed a strong preference for mean water column velocities of 0.36–0.69 m/s and depths of 0.38–0.84 m. The substrate range preferred for O. mykiss spawning was within 32–90 mm. O. mykiss spawning microhabitat predictions passed multiple bioverification tests enabling development of a habitat area versus discharge relation that showed that the lower Yuba River has ample O. mykiss spawning habitat.

Use of streambed substrate as refuge by steelhead or rainbow trout Oncorhynchus mykiss during simulated freshets.

A flume was used to estimate the carrying capacity of streambed substrates for juvenile steelhead or rainbow trout Oncorhynchus mykiss seeking refuge from simulated freshets. The simulated freshets had mean water column velocities of c. 1·1 m s(-1). The number of O. mykiss finding cover within the interstices of the substrate was documented for different substrate sizes and levels of embeddedness. The availability of suitable refuges determined the carrying capacity of the substrate for O. mykiss. For the size of the O. mykiss tested [mean ± s.d. fork length (L(F)) = 122 ± 12.6 mm], the number of interstices with depths ≥200 mm measured with a 14.0 mm diameter flexible plastic tube was the best predictor of the number of O. mykiss able to find cover (r(2) = 0.75). Oncorhynchus mykiss seeking refuge from freshets may need deeper interstices than those seeking concealment at autumn or winter base flows. The availability of interstices suitable as refuge from high flows may determine autumn and winter carrying capacity.

Spatial and Temporal Distribution of Spawning Events and Habitat Characteristics of Sacramento River Green Sturgeon

AbstractSpawning of the Southern Distinct Population Segment of Green Sturgeon Acipenser medirostris occurs annually within the Sacramento River in California. Artificial substrate samplers were used to collect Green Sturgeon eggs between 2008 and 2012 and in a reach of the river 94 river kilometers (rkm) long (rkm 426–332). A total of 268 eggs and 5 posthatch larvae were sampled from seven identified spawning sites between April 2 and July 7, primarily from medium gravel substrates. At these sites the mean water column velocities were 0.8 m/s at depths ranging from 0.6 to 11.3 m (6.4 ± 2.3 m, mean ± SD). We noted an average discharge of 314 m3/s and a median turbidity value of 3.9 NTU during estimated spawning events. Spawning at all sites occurred when average water temperatures were 13.5 ± 1.0°C and during water year types ranging from critically dry to wet. Green Sturgeon eggs averaged 4.11 ± 0.20 mm in diameter (n = 207), were very adhesive, and were between developmental stages 2 (just fertilized) and 44 (posthatch larva). We estimated that eggs were collected from a minimum of 54 different spawning events, based on sample date and location, egg developmental stage at capture, and water temperatures. Green Sturgeon spawning data indicates there is spatial separation from sympatric White Sturgeon A. transmontanus, but some temporal overlap exists. The thermally and hydrologically managed Sacramento River with its numerous diversions and competing water demands appears to have an approximate reach of 120 rkm in the 405‐km‐long river that is favorable for Green Sturgeon spawning in most years. Management decisions need to assess and incorporate the spawning habitat requirements of Green Sturgeon and coordinate this information with that of endangered winter‐run Chinook Salmon Oncorhynchus tshawytscha while attempting to meet the diverse demands of the limited Sacramento River water resources.

An experimental test and models of drift and dispersal processes of pallid sturgeon (Scaphirhynchus albus) free embryos in the Missouri River

Free embryos of wild pallid sturgeon Scaphirhynchus albus were released in the Missouri River and captured at downstream sites through a 180-km reach of the river to examine ontogenetic drift and dispersal processes. Free embryos drifted primarily in the fastest portion of the river channel, and initial drift velocities for all age groups (mean = 0.66–0.70 m s−1) were only slightly slower than mean water column velocity (0.72 m s−1). During the multi-day long-distance drift period, drift velocities of all age groups declined an average of 9.7% day−1. Younger free embryos remained in the drift upon termination of the study; whereas, older age groups transitioned from drifting to settling during the study. Models based on growth of free embryos, drift behavior, size-related variations in drift rates, and channel hydraulic characteristics were developed to estimate cumulative distance drifted during ontogenetic development through a range of simulated water temperatures and velocity conditions. Those models indicated that the average free embryo would be expected to drift several hundred km during ontogenetic development. Empirical data and model results highlight the long-duration, long-distance drift and dispersal processes for pallid sturgeon early life stages. In addition, results provide a likely mechanism for lack of pallid sturgeon recruitment in fragmented river reaches where dams and reservoirs reduce the length of free-flowing river available for pallid sturgeon free embryos during ontogenetic development.

Habitat Selection and Overlap of Atlantic Salmon and Smallmouth Bass Juveniles in Nursery Streams

AbstractIntroduced smallmouth bass Micropterus dolomieu have invaded much of the historic freshwater habitat of Atlantic salmon Salmo salar in North America, yet little is known about the ecological interactions between the two species. We investigated the possibility of competition for habitat between age‐0 Atlantic salmon and age‐0 and age‐1 smallmouth bass by means of in situ observations and a mesocosm experiment. We used snorkel observation to identify the degree and timing of overlap in habitat use in our in situ observations and to describe habitat shifts by Atlantic salmon in the presence of smallmouth bass in our mesocosm experiments. In late July 2008, we observed substantial overlap in the depths and mean water column velocities used by both species in sympatric in situ conditions and an apparent shift by age‐0 Atlantic salmon to shallower water that coincided with the period of high overlap. In the mesocosm experiments, we detected no overlap or habitat shifts by age‐0 Atlantic salmon in the presence age‐1 smallmouth bass and low overlap and no habitat shifts of Atlantic salmon and age‐0 smallmouth bass in fall 2009. In 2009, summer floods with sustained high flows and low temperatures resulted in the nearly complete reproductive failure of the smallmouth bass in our study streams, and we did not observe a midsummer habitat shift by Atlantic salmon similar to that seen in 2008. Although this prevented us from replicating our 2008 experiments under similar conditions, the virtual year‐class failure of smallmouth bass itself is enlightening. We suggest that future studies incorporate the effects of varying temperature and discharge to determine how abiotic factors affect the interactions between these species and thus mediate the outcomes of potential competition.

Anchoring Submersible Ultrasonic Receivers in River Channels with Stable Substrate

AbstractWe developed an anchoring system for submersible ultrasonic receivers (SURs) that we placed on the bottom of the riverine reaches of three main‐stem reservoirs in the upper Tennessee River. Each anchor consisted of a steel tube (8.9 × 35.6 cm) welded vertically to a round plate of steel (5.1 × 40.6 cm). All seven SURs and their 57‐kg anchors were successfully deployed and retrieved three times over 547 d by a dive team employing surface air‐breathing equipment and a davit‐equipped boat. All of the anchors and their SURs remained stationary over two consecutive winters on the hard‐bottom, thalweg sites where they were deployed. The SUR and its anchor at the most downriver site experienced flows that exceeded 2,100 m3/s and mean water column velocities of about 0.9 m/s.

Microhabitat use by three endemic Iberian cyprinids in Mediterranean rivers (Tagus River Basin, Spain)

Abstract Microhabitat use by three endemic Iberian cyprinids, Barbus bocagei (Steindachner), Pseudochondrostoma polylepis (Steindachner), and Squalius pyrenaicus (Günther) was studied in terms of depth, mean water column velocity, focal height, focal velocity, distance to shore and substrate. Data were obtained by snorkelling during spring and summer at nine sites of the Tagus River Basin, Spain. Habitat suitability criteria (HSC) were calculated, including fish position and focal velocity in the water column. Species comparison showed differences in depth and focal height (indicating a vertical segregation), and greater water velocities for Pseudochondrostoma. Size‐class comparisons mainly showed differences in depth and focal height (correlated with fish size). The fish groups (3 species × 3 length classes) were assigned to microhabitat functional types. The results are essential for environmental flow assessments and allow 2‐ and 3‐dimensional habitat simulations in Mediterranean rivers; they are also useful to define critical habitats for the conservation of native fish populations.

Drift Dynamics of Larval Pallid Sturgeon and Shovelnose Sturgeon in a Natural Side Channel of the Upper Missouri River, Montana

AbstractThe drift dynamics of larval shovelnose sturgeon Scaphirhynchus platorynchus (1, 2, 6, and 10 d posthatch (dph)) and pallid sturgeon S. albus (1, 2, 5, 9, 11, and 17 dph) were examined in a natural side channel of the Missouri River to quantify the vertical drift location of larvae in the water column, determine the drift velocity of larvae relative to water velocity, and simulate the cumulative distance (km) drifted by larvae during ontogenetic development. Larvae were released at the side‐channel inlet and sampled at points 100, 500, 900, and 1,300 m downstream. Larvae drifted primarily near the riverbed, as 58–79% of recaptured shovelnose sturgeon and 63–89% of recaptured pallid sturgeon were sampled in the lower 0.5 m of the water column. The transition from the drifting to the benthic life stage was initiated at 6 dph (mean length, 15.6 mm) for shovelnose sturgeon and at 11–17 dph (mean length, 18.1–20.3 mm) for pallid sturgeon. Across ages, the drift rates of larval shovelnose sturgeon averaged 0.09–0.16 m/s slower than the mean water column velocity. The drift rates of pallid sturgeon were similar to or slightly slower (0.03–0.07 m/s) than the mean water column velocity for 1–11‐dph larvae. Conversely, 17‐dph larval pallid sturgeon dispersed downstream at a much slower rate (mean, 0.20 m/s slower than the mean water column velocity) owing to their transition to benthic habitats. Drift simulations indicated that the average larval shovelnose sturgeon may drift from 94 to 250 km and the average larval pallid sturgeon may drift from 245 to 530 km, depending on water velocity. Differences in larval drift dynamics between species provide a possible explanation for differences in recruitment between shovelnose sturgeon and pallid sturgeon in the upper Missouri River.

Estimation of preferred water flow parameters for four species of Simulium (Diptera: Simuliidae) in small clear streams in South Africa

Blackfly larvae typically occur in fast-flowing riffle sections of rivers, with different blackfly species showing preferences for different hydraulic conditions. Very little quantitative data exist on hydraulic conditions linked to the blackfly species occurring in South African streams. Stones-in-current biotopes (i.e. fast riffle flows over cobbles) were sampled from four sites in three small clear streams in the Eastern and Western Cape provinces of South Africa. Mean water column velocities at each sampled stone were measured using a mini current meter, while flow velocities closer to the boundary layer where blackfly larvae occurred were estimated using indirect techniques (standard hemispheres and aerating tablets). Standard hemispheres were also used to calculate more complex hydraulic parameters such as Froude and Reynolds numbers. Four species of Simuliid were sampled in sufficient numbers to show trends in flow velocity preferences. Simulium impukane and S. rutherfoordi both occurred at their highest densities at velocities of 0.3m s−1, while S. merops preferred velocities of 0.7m s−1. Simulium nigritarse SL attained the highest densities of all the blackfly species sampled, and its relative abundances were greatest at velocities of 0.8–0.9m s−1. Within the streams surveyed, all blackfly species occurred in subcritical-turbulent flows — based on a classification using Froude and Reynolds numbers — although two of the species were also found in high densities in supercritical flows where these existed at the sites. Local hydraulics within the stones-in-current biotope are complex, but in the absence of fine-scale equipment for measuring micro-velocities, standard hemispheres are a useful, cost-effective technique for the initial quantification of hydraulic parameters in small, clear streams. Such an approach facilitates further understanding of links between hydraulics and aquatic invertebrates in South African streams.

Habitat Restoration as a Means of Controlling Non‐Native Fish in a Mojave Desert Oasis

Abstract Non‐native fish generally cause native fish decline, and once non‐natives are established, control or elimination is usually problematic. Because non‐native fish colonization has been greatest in anthropogenically altered habitats, restoring habitat similar to predisturbance conditions may offer a viable means of non‐native fish control. In this investigation we identified habitats favoring native over non‐native fish in a Mojave Desert oasis (Ash Meadows) and used this information to restore one of its major warm water spring systems (Kings Pool Spring). Prior to restoration, native fishes predominated in warm water (25–32°C) stream and spring‐pool habitat, whereas non‐natives predominated in cool water (≤23°C) spring‐pool and marsh/slack water habitat. Native Amargosa pupfish (Cyprinodon nevadensis) and Ash Meadows speckled dace (Rhinichthys osculus nevadensis) inhabited significantly faster mean water column velocities (MWCV) and greater total depth (TD) than non‐native Sailfin molly (Poecilia latipinna) and Mosquitofish (Gambusia affinis) in warm water stream habitat, and Ash Meadows speckled dace inhabited significantly faster water than non‐natives in cool water stream habitat. Modification of the outflow of Kings Pool Spring from marsh to warm water stream, with MWCV, TD, and temperature favoring native fish, changed the fish composition from predominantly non‐native Sailfin molly and Mosquitofish to predominantly Ash Meadows pupfish. This result supports the hypothesis that restoring spring systems to a semblance of predisturbance conditions would promote recolonization of native fishes and deter non‐native fish invasion and proliferation.

Flexible Summer Habitat Selection by Wild, Allopatric Brown Trout in Lotic Environments

Abstract Spatial and temporal hydraulic heterogeneity influence distributional patterns of species in streams. Ecological theory suggests flexible habitat selection strategies are favored in such unstable environments. The effects of varying hydraulic conditions on habitat selection by brown trout Salmo trutta in summer were studied in eight streams in Norway and Scotland. At normal summer flows, brown trout averaging 16 cm total length (SD = ±5 cm, range = 3-43 cm) were selective in habitat use. The selection window was relatively narrow for focal water velocity (mean = 14 cm/s, median = 10 cm/s, 60.1% of observations ≤14 cm/s). Trout favored slower flowing pool areas, but selection ranges were wide (mean water column velocity = 24 ± 21 cm/s, range = 0-142 cm/s; mean depth = 69 ± 29 cm, range = 9-305 cm). Larger fish used deeper habitats; other variables did not correlate with size. Great overlaps in spatial niche used by the studied size-classes of trout indicated versatility in habitat selection (e.g.,...

Relations between complex hydraulics and the localized distribution of mussels in three regulated rivers

A stratified random sampling design was used to examine the relations between selected hydraulic variables and the density of unionids at five sites on the Green, Licking, and Rough Rivers of Kentucky. We located the strata to ensure that samples occurred in a wide range of hydraulic conditions at each site. Eight 0.25 m2 quadrat samples were collected from each 25 m2 stratum. We measured mean water column velocity, depth, and substrate roughness before we sampled each quadrat. ‘Fliesswasserstammtisch’ (FST) hemispheres were used to estimate shear stress. In all, we collected 798 individuals of 28 species of freshwater mussels. Simple hydraulic characteristics of our study sites were not correlated consistently with mussel density. For instance, water depth and mussel density were positively correlated in the Green River, negatively correlated in the Rough River, and not significantly correlated in the Licking River. In contrast, we found consistent negative correlations in all rivers between mussel density and complex hydraulic variables, such as shear velocity and FST hemisphere number. We believe that the limited recruitment observed in these rivers may have resulted from operation of upstream flood control dams that altered flow regimes seasonally. We suspect that the increased discharge during spring and early summer resulted in shear forces sufficiently high on mussel beds to prevent settlement of newly metamorphosed juveniles. Copyright © 2001 John Wiley & Sons, Ltd.

Relations between complex hydraulics and the localized distribution of mussels in three regulated rivers

A stratified random sampling design was used to examine the relations between selected hydraulic variables and the density of unionids at five sites on the Green, Licking, and Rough Rivers of Kentucky. We located the strata to ensure that samples occurred in a wide range of hydraulic conditions at each site. Eight 0.25 m2 quadrat samples were collected from each 25 m2 stratum. We measured mean water column velocity, depth, and substrate roughness before we sampled each quadrat. ‘Fliesswasserstammtisch’ (FST) hemispheres were used to estimate shear stress. In all, we collected 798 individuals of 28 species of freshwater mussels. Simple hydraulic characteristics of our study sites were not correlated consistently with mussel density. For instance, water depth and mussel density were positively correlated in the Green River, negatively correlated in the Rough River, and not significantly correlated in the Licking River. In contrast, we found consistent negative correlations in all rivers between mussel density and complex hydraulic variables, such as shear velocity and FST hemisphere number. We believe that the limited recruitment observed in these rivers may have resulted from operation of upstream flood control dams that altered flow regimes seasonally. We suspect that the increased discharge during spring and early summer resulted in shear forces sufficiently high on mussel beds to prevent settlement of newly metamorphosed juveniles. Copyright © 2001 John Wiley & Sons, Ltd.

Spawning Habitat Used by Fall Chinook Salmon in the Snake River

Literature describing spawning habitat used by fall-run chinook salmon Oncorhynchus tshawytscha is lacking for populations using large, main-stem rivers. A stable spawning flow regime in the Snake River below Hells Canyon Dam and enhanced survey capabilities using remote underwater videography allowed us to accurately describe spawning habitat used by fall chinook salmon within this large river. Water depth measured at 205 fall chinook salmon redds ranged from 0.2 to 6.5 m. Mean water column velocity at 145 redds ranged from 0.4 to 2.1 m/s, and substrate- level water velocity at 164 redds ranged from 0.1 to 2.0 m/s. Substrate size classifications from 112 redds indicated that areas having particle sizes that are relatively homogenous with diameters of 2.5–15.0 cm are used for spawning. During our study, spawning generally began as water temperatures dropped below 16.0°C, and concluded as temperatures approached 5.0°C. Our results corroborate earlier observations of fall chinook salmon spawning in deep, fast-velocity water in the Hanford Reach of the Columbia River and significantly expand current criteria used to model spawning habitat availability for these fish within large rivers.

Effect of stream microhabitat characteristics on rate of net energy gain by juvenile smallmouth bass, Micropterus dolomieu

We estimated the rate of energy consumed and expended by 43 juvenile smallmouth bass, Micropterus dolomieu, videotaped in the North Anna River during 1991. Habitat data were also collected in the areas where each individual was videotaped. The net rate of energy gained (J·min−1) increased as water depths decreased and mean water-column velocities increased. This relationship remained statistically significant even when we assumed that consumption was as little as 40% of our original estimates, and that respiratory costs were three times higher than we suspected. The net rate of energy gained by juvenile smallmouth bass was only affected by the characteristics of the area that they searched and not the foraging tactics they employed.

Near‐substrate hydraulic conditions under artificial floods from peaking hydropower operation: A preliminary analysis of disturbance intensity and duration

AbstractFST hemispheres of identical size but varying density were used to evaluate the intensity and duration of near‐bottom shear stresses during peaking hydropower floods of various magnitudes (maximum 345 m3 s−1; baseflow 2.75 m3 s−1). The FST values at single points were recorded at rapid intervals along previously surveyed transects during the rising limb of flood events at various distances from the hydropower facility. Whole transect evaluations were ‐accomplished after the ‘flood’ level had stabilized.Measurements of over 1300 FST values were significantly correlated with mean water column velocity and complex hydraulic conditions such as turbulence, shear velocity and viscous sublayer. FST hemispheres are a rapid means of characterizing hydraulic conditions in medium order rivers. Changes in near‐substrate hydraulic conditions were patchy and could be classified in terms of risk to fauna of either dislodgement or physical damage. Low risk (class 1) changes occurred when FST values increased rapidly but returned to initial values after a short period of time (< 90 minutes) from the arrival of the peaking wave. Medium risk (class 2) changes occurred when FST values increased rapidly and decreased after a short period of time, but remained at levels higher than before the arrival of the flood wave. High risk (class 3) changes occurred when FST values increased rapidly and remained at high levels during the entire flood event.It is suggested that floods that do not initiate significant bed movement create a mosaic of patches of variable disturbance risk to benthic fauna. Rapid evaluation by FST hemispheres could be used to characterize the overall disturbance potential for the channel by the construction of a weighted index based on the frequency of the various risk classes occurring in the reach being evaluated. A more appropriate analysis of impacts of peaking hydropower and other flood events must include the existence of hydraulic refugia and the movement patterns of both benthic species and those species that occupy the water column.

Habitat selection by rainbow trout Oncorhynchus mykiss in a California stream: implications for the Instream Flow Incremental Methodology

AbstractWe quantified microhabitat selection of rainbow trout (Oncorhynchus mykiss) at 2 flows (low= 1.13 m3. s−1 and high =4.95 m3. s−1) in the Pit River, California. Flows were controlled by an upstream dam and habitat availability was similar during 4 sampling periods at low flow and 2 periods at high flow. A principal components analysis reduced 6 microhabitat variables to 3 new variables that explained 80% of the observed variance. The 3 components loaded heavily on velocity variables, depth variables and substrate. Microhabitat selection generally differed among macrohabitats (i. e., pools, runs, and riffles). Rainbow trout selected different microhabitats at high flow relative to low flow in response to the availability of deeper, faster water. At low flow, depth and velocity selection were positively correlated with seasonal temperature change for adults but not juveniles. Rainbow trout apparently sought shelter in interstitial spaces in the substrate of runs and riffles during the day in early winter. Generally, large rainbow trout were observed in pools, intermediate‐sized fish in runs, and small trout in riffles. The largest fish occurred in slow, deep areas of pools, where they moved slowly without orientation to flow and were not observed feeding, whereas small fish generally faced upstream and fed in all habitat types. Foraging forays were directed up in the water column at velocities similar to the mean water column velocities at holding positions. Rainbow trout were the most abundant species in 76% of the population survey stations. Other species that might have influenced microhabitat selection by rainbow trout were uncommon.

Microhabitat Use by Stream-Resident Brown Trout: Bioenergetic Consequences

Abstract Seasonal shifts in the microhabitats used by feeding brown trout Salmo trutta in a northern Spanish river were related to changes in habitat availability. At higher flows, mean water column velocities, focal velocities, and depths occupied by brown trout were greater and focal elevations above the substratum were less, but no changes in substratum use were detected. Cover use differed between dates, apparently according to the presence of suitable materials. In May, a lower proportion of the observed fish were holding feeding positions than in July, October, or January. Total metabolic costs associated with holding feeding stations were primarily determined by swimming costs, and both costs varied substantially between dates, mainly as a consequence of the different swimming speeds of brown trout on the different dates. Standard metabolism was less important than swimming cost in total energy expenditure and showed no obvious relationship with total metabolic rate. Brown trout exhibited considera...

Reproductive biology of an endemic, amphidromous goby Lentipes concolor in Hawaiian streams

Egg size, fecundity, nest site selection, and breeding structure of the amphidromous Hawaiian goby Lentipes concolor were studied to determine if there was any relationship between these aspects of the breeding biology of this fish and the very long larval period of Hawaiian stream gobies. To do this, these parameters were compared with those known for other gobioid fishes. While eggs were small and females were found to lay up to four nests in a season, reproductive biology and mating behavior of L. concolor were typical of many gobies in spite of its amphidromous life history pattern. Lentipes concolor nests were found from October to June with a spawning peak in February. Nests tended to be positioned closer to the stream bank, and under larger rocks than if they were situated randomly. Depth and mean water column velocity over nest sites did not differ from that at randomly selected points in the stream.

Spawning and Rearing Habitat Use by White Sturgeons in the Columbia River Downstream from McNary Dam

Spawning and rearing habitats used by white sturgeons Acipenser transmontanus were described from water temperature, depth, and velocity measurements and substrate types present at sites where eggs, larvae, young-of-the-year, and juveniles (ages 1–7) were collected. Spawning and egg incubation occurred in the swiftest water available (mean water column velocity, 0.8–2.8 m/s), which was within 8 km downstream from each of the four main-stem Columbia River dams in our study area. Substrates where spawning occurred were mainly cobble, boulder, and bedrock. Yolk-sac larvae were transported by the river currents from spawning areas into deeper areas with lower water velocities and finer substrates. Young-of-the-year white sturgeons were found at depths of 9–57 m, at mean water column velocities of 0.6 m/s and less, and over substrates of hard clay, mud and silt, sand, gravel, and cobble. Juvenile fish were found at depths of 2–58 m, at mean water column velocities of 1.2 m/s and less, and over substrates of hard clay, mud and silt, sand, gravel, cobble, boulder, and bedrock.