Understanding Water Quality Parameters
What it is: Dissolved oxygen (DO) is the amount of oxygen gas dissolved in water — the same O₂ molecule that's in the air, but in solution. Most aquatic animals breathe it through gills, lungs, or skin. The sensor measures this in milligrams of oxygen per litre of water, with 0 meaning anoxic (no oxygen) and around 8–10 mg/L meaning fully aerated freshwater at typical temperatures. Cold water holds more dissolved oxygen than warm water; salty water holds less than fresh.
Why it matters: DO is one of the single most important indicators of aquatic ecosystem health. Fish, crabs, worms, molluscs, prawns and other aquatic life need oxygen to survive. Low oxygen levels can lead to stress or death for these organisms. Different species have different tolerance ranges.
What affects it: DO rises when plants photosynthesise (during the day) and when fast-flowing water mixes with air. DO falls when bacteria break down organic matter — consuming oxygen in the process. This can happen during "blackwater events" — when major floods inundate the floodplain and decomposing organic material consumes the oxygen in the floodwater. DO can also fall due to chemical reactions resulting from acid sulfate soil drainage (e.g. oxidation of Fe²⁺, which consumes oxygen). Temperature is a major driver: cool water absorbs more, warming water releases it. DO often follows a daily cycle, peaking in the afternoon when plants are most active and dropping overnight.
What it is: Where dissolved oxygen (mg/L) measures the absolute amount of oxygen in the water, DO% measures how full the water is relative to what it could hold. 100% means the water is in equilibrium with the atmosphere — holding as much oxygen as it can dissolve at that temperature and salinity. Above 100% means the water is supersaturated; below 100% means it's undersaturated. The two numbers tell complementary stories: DO in mg/L is what's actually available to organisms; DO% reveals what biological and physical processes are doing to the water.
Why it matters: DO% removes the temperature effect that makes raw DO hard to compare across seasons or sites.
What affects it: The same as for DO mg/L.
What it is: pH measures how acidic or alkaline water is, on a scale from 0 (strongly acidic, like battery acid) to 14 (strongly alkaline, like bleach), with 7 being neutral. River water typically sits between 6.5 and 8.5. The scale is logarithmic: a one-unit change represents a ten times change in acidity. A river at pH 5 is ten times more acidic than one at pH 6, and a hundred times more acidic than one at pH 7.
Why it matters: Most freshwater fish, invertebrates, and plants are adapted to a relatively narrow pH band — roughly 6.5 to 8.5. Outside that range, gill function is impaired, eggs fail to develop, immune systems are compromised, and the toxicity of metals like aluminium increases sharply. Sudden pH swings, even within the "safe" range, can stress fish and trigger die-offs.
What affects it: Photosynthesis raises pH (plants consume CO₂, which is acidic in water), so pH typically rises through the day and falls overnight, often by 0.5–1.0 units. Rainfall is naturally slightly acidic. On the estuary floodplain, drains that intersect acid-sulfate soils can transport acid and metal-rich water into the river with pH well below 5.5.
What it is: ORP measures the electrochemical balance of the water — whether dissolved compounds are in an oxidised (electron-poor) or reduced (electron-rich) state. The sensor uses a platinum electrode against a reference, reading in millivolts. Positive values (typically +200 to +400 mV) indicate well-oxygenated, oxidising conditions; values near zero indicate transitional conditions; strongly negative values (−100 to −400 mV) indicate reducing, oxygen-depleted conditions where sulfide, methane, and reduced iron can form.
Why it matters: ORP is a sensitive early indicator of biogeochemical condition. As bacteria consume oxygen, they progressively use other electron acceptors (nitrate, manganese, iron, sulfate, CO₂), each step driving ORP lower. By the time DO has dropped to zero, ORP has often already revealed the trajectory. ORP also controls the toxicity of many compounds: under reducing conditions, dissolved iron and manganese mobilise from sediments; hydrogen sulfide (highly toxic to fish) can form; methylmercury production accelerates.
What affects it: DO concentration is a dominant driver — high DO means high ORP, oxygen depletion drops it. Organic loading accelerates the drop. Low pH water draining from acid sulfate soils will often have a much higher ORP signature (>300 mV).
What it is: The temperature of the river water, measured in degrees Celsius. This is the most fundamental water quality measurement — almost every other parameter (DO, pH, biological activity, chemical reaction rates) depends on it.
Why it matters: Aquatic species are cold-blooded — their metabolism, growth, breeding, and survival are tied directly to water temperature. Australian bass spawn in estuaries when temperatures drop in winter; sea mullet migrate based on temperature cues; freshwater shrimp are sensitive to summer extremes. Warmer water also holds less dissolved oxygen and accelerates the decomposition that consumes it, so temperature spikes can trigger oxygen crises.
What affects it: Daily air temperature, solar radiation, shading by riparian vegetation, river depth (shallow water heats and cools faster), and groundwater inflows (which buffer temperature).
What it is: Salinity is the total amount of dissolved salt in the water — sodium, chloride, calcium, magnesium, sulfate, and others. Units are ppt — parts per thousand, or grams of salt per kilogram of water. Freshwater is below 0.5 ppt; full seawater is around 35 ppt; the brackish zone in between is where estuaries live.
Why it matters: Salinity defines which species can live where. Freshwater fish, crustaceans, and invertebrates struggle once salinity exceeds about 2 ppt; estuarine and marine species need it well above that. In a tidal coastal river like the Clarence, the salt wedge shifts up and down the river daily with the tide, and up and down between seasons with rainfall. Tracking salinity reveals where these ecological zones currently sit and how they're moving.
What affects it: Tides push salt upstream; freshwater inflows push it back down. Drought reduces freshwater inflow and allows salt to migrate further upstream — sometimes by many kilometres. Heavy rainfall flushes the system seaward.
What it is: Turbidity measures how cloudy or murky the water is — specifically, how much light is scattered by suspended particles, often fine sediment. The probe sensor shines a beam of light into the water and measures how much bounces back from a defined angle (90°), giving a reading in NTU (Nephelometric Turbidity Units).
Why it matters: Turbidity tells you what's being carried in the water. High turbidity reduces light penetration, which suppresses photosynthesis by aquatic plants and algae. Suspended sediment clogs fish gills, smothers spawning gravels, and can carry phosphorus and pesticides that adsorb to particle surfaces.
What affects it: Rainfall and runoff are the dominant drivers — bare soil, bare paddocks, eroding banks and unsealed roads can all contribute. Flocculation of iron oxides from acid sulfate soil drainage can also increase turbidity, as can organic-rich "black water" draining from the floodplains after floods. Wind and boat wakes can stir up bottom sediments in shallow reaches. After major rainfall, turbidity can often stay elevated for many days as fine clay particles slowly settle.