To solve this problem, we need to understand the impact of domestic sewage rich in organic waste on a river ecosystem.$\newline$Let's analyze the options:$\newline \bullet$Option 1: Drying of the river very soon due to algal bloom.$\newline$ - Algal blooms can occur due to nutrient enrichment, but they do not cause the river to dry up.$\newline$ - This option is incorrect.$\newline \bullet$Option 2: Increased population of aquatic food web organisms.$\newline$ - While nutrients can initially increase some populations, the overall effect of organic waste is often negative due to oxygen depletion.$\newline$ - This option is incorrect.$\newline \bullet$Option 3: An increased production of fish due to biodegradable nutrients.$\newline$ - Although nutrients can support fish growth, excessive organic waste leads to oxygen depletion, harming fish.$\newline$ - This option is incorrect.$\newline \bullet$Option 4: Death of fish due to lack of oxygen.$\newline$ - Organic waste increases microbial activity, consuming oxygen and leading to hypoxic conditions.$\newline$ - This results in the death of fish and other aquatic organisms.$\newline$ - This option is correct.$\newline$Therefore, the correct answer is Option 4: Death of fish due to lack of oxygen.